diff --git a/src/app/features/tasks/store/task.actions.ts b/src/app/features/tasks/store/task.actions.ts
index 44941b5872..ce0d68fa6b 100644
--- a/src/app/features/tasks/store/task.actions.ts
+++ b/src/app/features/tasks/store/task.actions.ts
@@ -37,9 +37,20 @@ export const __updateMultipleTaskSimple = createAction(
   }),
 );
 
+// NOTE: despite the "Ui" name this carries _hideSubTasksMode, which is task
+// data that must survive a restart. It is persisted (op-log) like updateTask so
+// the collapse state is not lost on reload. See issue #8781.
 export const updateTaskUi = createAction(
   '[Task] Update Task Ui',
-  props<{ task: Update<Task> }>(),
+  (taskProps: { task: Update<Task> }) => ({
+    ...taskProps,
+    meta: {
+      isPersistent: true,
+      entityType: 'TASK',
+      entityId: taskProps.task.id as string,
+      opType: OpType.Update,
+    } satisfies PersistentActionMeta,
+  }),
 );
 
 export const removeTagsForAllTasks = createAction(
@@ -47,11 +58,6 @@ export const removeTagsForAllTasks = createAction(
   props<{ tagIdsToRemove: string[] }>(),
 );
 
-export const toggleTaskHideSubTasks = createAction(
-  '[Task] Toggle Show Sub Tasks',
-  props<{ taskId: string; isShowLess: boolean; isEndless: boolean }>(),
-);
-
 export const moveSubTask = createAction(
   '[Task] Move sub task',
   (taskProps: {
diff --git a/src/app/features/tasks/store/task.reducer.spec.ts b/src/app/features/tasks/store/task.reducer.spec.ts
index 98ae25be1c..d66e73d3d5 100644
--- a/src/app/features/tasks/store/task.reducer.spec.ts
+++ b/src/app/features/tasks/store/task.reducer.spec.ts
@@ -1,6 +1,7 @@
 /* eslint-disable @typescript-eslint/naming-convention */
-import { Task, TaskDetailTargetPanel, TaskState } from '../task.model';
+import { HideSubTasksMode, Task, TaskDetailTargetPanel, TaskState } from '../task.model';
 import { initialTaskState, taskReducer } from './task.reducer';
+import { convertOpToAction } from '../../../op-log/apply/operation-converter.util';
 import * as fromActions from './task.actions';
 import { TaskSharedActions } from '../../../root-store/meta/task-shared.actions';
 import { INBOX_PROJECT } from '../../project/project.const';
@@ -11,6 +12,7 @@ import {
 import { _resetDevErrorState } from '../../../util/dev-error';
 import { PlannerActions } from '../../planner/store/planner.actions';
 import { loadAllData } from '../../../root-store/meta/load-all-data.action';
+import { ActionType, OpType, Operation } from '../../../op-log/core/operation.types';
 
 describe('Task Reducer', () => {
   const createTask = (id: string, partial: Partial<Task> = {}): Task => ({
@@ -1114,4 +1116,69 @@ describe('Task Reducer', () => {
       expect(() => taskReducer(stateWithUndefined, action)).not.toThrow();
     });
   });
+
+  // Regression: subtask collapse state (_hideSubTasksMode) must survive a restart.
+  // It only persists if the action that writes it is captured to the op-log,
+  // which requires isPersistent metadata. `updateTaskUi` carries an absolute
+  // value (replay-safe); toggleSubTaskMode resolves the value and dispatches it.
+  // See issue #8781.
+  describe('updateTaskUi persistence metadata', () => {
+    it('should be a persistent TASK Update action', () => {
+      const action = fromActions.updateTaskUi({
+        task: { id: 'task1', changes: { _hideSubTasksMode: undefined } },
+      });
+
+      expect(action.meta).toBeDefined();
+      expect(action.meta.isPersistent).toBe(true);
+      expect(action.meta.entityType).toBe('TASK');
+      expect(action.meta.entityId).toBe('task1');
+      expect(action.meta.opType).toBe(OpType.Update);
+    });
+
+    // The metadata assertion above proves the change is *eligible* for capture.
+    // This one proves it actually survives the whole op-log path end to end:
+    // dispatch -> capture into an operation payload -> serialize -> convert the
+    // op back to an action -> replay through the reducer. The JSON serialize hop
+    // mirrors the sync transport and the SQLite op-log backend, both of which
+    // round-trip op payloads as JSON, so it is where a lost value would regress.
+    // See issue #8781.
+    it('should round-trip _hideSubTasksMode through capture, serialization and replay', () => {
+      const stateShown: TaskState = {
+        ...initialTaskState,
+        ids: ['task1'],
+        entities: { task1: createTask('task1') },
+      };
+
+      // 1. Dispatch: the persistent action toggleSubTaskMode dispatches on collapse.
+      const action = fromActions.updateTaskUi({
+        task: {
+          id: 'task1',
+          changes: { _hideSubTasksMode: HideSubTasksMode.HideAll },
+        },
+      });
+
+      // 2. Capture: the effects store the action fields under payload.actionPayload.
+      const op: Operation = {
+        id: 'op-8781',
+        actionType: action.type as ActionType,
+        opType: action.meta.opType,
+        entityType: action.meta.entityType,
+        entityId: action.meta.entityId as string,
+        payload: { actionPayload: { task: action.task }, entityChanges: [] },
+        clientId: 'clientA',
+        vectorClock: { clientA: 1 },
+        timestamp: 0,
+        schemaVersion: 1,
+      };
+
+      // 3. Serialize over the wire / into the op-log, then read it back.
+      const wireOp = JSON.parse(JSON.stringify(op)) as Operation;
+
+      // 4. Convert the persisted op back into a replayable action and replay it.
+      const replayAction = convertOpToAction(wireOp);
+      const replayed = taskReducer(stateShown, replayAction);
+
+      expect(replayed.entities.task1?._hideSubTasksMode).toBe(HideSubTasksMode.HideAll);
+    });
+  });
 });
diff --git a/src/app/features/tasks/store/task.reducer.ts b/src/app/features/tasks/store/task.reducer.ts
index a09e13d0a3..dd06fe8a80 100644
--- a/src/app/features/tasks/store/task.reducer.ts
+++ b/src/app/features/tasks/store/task.reducer.ts
@@ -12,7 +12,6 @@ import {
   setCurrentTask,
   setSelectedTask,
   toggleStart,
-  toggleTaskHideSubTasks,
   unsetCurrentTask,
   updateTaskUi,
 } from './task.actions';
@@ -362,49 +361,6 @@ export const taskReducer = createReducer<TaskState>(
     return taskAdapter.updateMany(tasks, state);
   }),
 
-  // TODO simplify
-  on(toggleTaskHideSubTasks, (state, { taskId, isShowLess, isEndless }) => {
-    const task = getTaskById(taskId, state);
-    const subTasks = task.subTaskIds.map((id) => getTaskById(id, state));
-    const doneTasksLength = subTasks.filter((t) => t.isDone).length;
-    const isDoneTaskCaseNeeded = doneTasksLength && doneTasksLength < subTasks.length;
-    // for easier calculations we use 0 instead of undefined for show state
-    const oldVal = task._hideSubTasksMode || 0;
-    let newVal: number = isShowLess ? oldVal + 1 : oldVal - 1;
-
-    if (!isDoneTaskCaseNeeded && newVal === 1) {
-      if (isShowLess) {
-        newVal = 2;
-      } else {
-        newVal = 0;
-      }
-    }
-
-    if (isEndless) {
-      if (newVal < 0) {
-        newVal = 2;
-      } else if (newVal > 2) {
-        newVal = 0;
-      }
-    } else {
-      if (newVal < 0) {
-        newVal = 0;
-      } else if (newVal > 2) {
-        newVal = 2;
-      }
-    }
-
-    return taskAdapter.updateOne(
-      {
-        id: taskId,
-        changes: {
-          _hideSubTasksMode: newVal || undefined,
-        },
-      },
-      state,
-    );
-  }),
-
   on(moveSubTask, (state, { taskId, srcTaskId, targetTaskId, afterTaskId }) => {
     // Guard against invalid moves (e.g. 'UNDONE' passed as ID) that may
     // appear in older op-log entries. Also reject self-moves where the
diff --git a/src/app/features/tasks/task.service.ts b/src/app/features/tasks/task.service.ts
index 27f5c02bae..3c595319ed 100644
--- a/src/app/features/tasks/task.service.ts
+++ b/src/app/features/tasks/task.service.ts
@@ -30,10 +30,10 @@ import {
   setCurrentTask,
   setSelectedTask,
   toggleStart,
-  toggleTaskHideSubTasks,
   unsetCurrentTask,
   updateTaskUi,
 } from './store/task.actions';
+import { getNextHideSubTasksMode } from './util/get-next-hide-sub-tasks-mode';
 import { IssueProviderKey } from '../issue/issue.model';
 import { GlobalTrackingIntervalService } from '../../core/global-tracking-interval/global-tracking-interval.service';
 import { BatchedTimeSyncAccumulator } from '../../core/util/batched-time-sync-accumulator';
@@ -1156,7 +1156,28 @@ export class TaskService {
     isShowLess: boolean = true,
     isEndless: boolean = false,
   ): void {
-    this._store.dispatch(toggleTaskHideSubTasks({ taskId, isShowLess, isEndless }));
+    const entities = this._taskEntities();
+    const task = entities[taskId];
+    if (!task) {
+      return;
+    }
+    const subTasks = task.subTaskIds
+      .map((id) => entities[id])
+      .filter((t): t is Task => !!t);
+    const doneCount = subTasks.filter((t) => t.isDone).length;
+    // Persist the resolved absolute value via updateTaskUi (replay-safe) rather
+    // than a relative toggle command, so the collapse state survives a restart.
+    // Replaying a relative command would recompute from live state and diverge
+    // across devices. See issue #8781.
+    this.updateUi(taskId, {
+      _hideSubTasksMode: getNextHideSubTasksMode(
+        task._hideSubTasksMode,
+        doneCount,
+        subTasks.length,
+        isShowLess,
+        isEndless,
+      ),
+    });
   }
 
   hideSubTasks(id: string): void {
diff --git a/src/app/features/tasks/util/get-next-hide-sub-tasks-mode.spec.ts b/src/app/features/tasks/util/get-next-hide-sub-tasks-mode.spec.ts
new file mode 100644
index 0000000000..1444fd5ae8
--- /dev/null
+++ b/src/app/features/tasks/util/get-next-hide-sub-tasks-mode.spec.ts
@@ -0,0 +1,75 @@
+import { getNextHideSubTasksMode } from './get-next-hide-sub-tasks-mode';
+import { HideSubTasksMode } from '../task.model';
+
+const { HideDone, HideAll } = HideSubTasksMode;
+
+describe('getNextHideSubTasksMode', () => {
+  describe('with a mix of done and undone subtasks (HideDone step is available)', () => {
+    // 3 subtasks, 1 done → isDoneTaskCaseNeeded
+    const next = (
+      cur: HideSubTasksMode | undefined,
+      isShowLess: boolean,
+      isEndless = false,
+    ): HideSubTasksMode | undefined =>
+      getNextHideSubTasksMode(cur, 1, 3, isShowLess, isEndless);
+
+    it('shows-less stepping: show → HideDone → HideAll (then clamps)', () => {
+      expect(next(undefined, true)).toBe(HideDone);
+      expect(next(HideDone, true)).toBe(HideAll);
+      expect(next(HideAll, true)).toBe(HideAll);
+    });
+
+    it('shows-more stepping: HideAll → HideDone → show (then clamps)', () => {
+      expect(next(HideAll, false)).toBe(HideDone);
+      expect(next(HideDone, false)).toBeUndefined();
+      expect(next(undefined, false)).toBeUndefined();
+    });
+  });
+
+  describe('without a mix (no done, or all done) — HideDone step is skipped', () => {
+    // 3 subtasks, 0 done → not isDoneTaskCaseNeeded
+    const next = (
+      cur: HideSubTasksMode | undefined,
+      isShowLess: boolean,
+      isEndless = false,
+    ): HideSubTasksMode | undefined =>
+      getNextHideSubTasksMode(cur, 0, 3, isShowLess, isEndless);
+
+    it('shows-less jumps straight from show to HideAll (skipping HideDone)', () => {
+      expect(next(undefined, true)).toBe(HideAll);
+    });
+
+    it('shows-more jumps straight from HideAll to show (skipping HideDone)', () => {
+      expect(next(HideAll, false)).toBeUndefined();
+    });
+
+    it('treats all-done the same as none-done (no HideDone step)', () => {
+      expect(getNextHideSubTasksMode(undefined, 3, 3, true, false)).toBe(HideAll);
+    });
+  });
+
+  describe('endless wrap-around', () => {
+    const next = (
+      cur: HideSubTasksMode | undefined,
+      isShowLess: boolean,
+    ): HideSubTasksMode | undefined =>
+      getNextHideSubTasksMode(cur, 0, 3, isShowLess, true);
+
+    it('wraps HideAll → show when stepping past the hide end', () => {
+      expect(next(HideAll, true)).toBeUndefined();
+    });
+
+    it('wraps show → HideAll when stepping past the show end', () => {
+      expect(next(undefined, false)).toBe(HideAll);
+    });
+  });
+
+  it('returns undefined (not 0) when clearing collapse state', () => {
+    // decrement from HideAll with no mix clears back to fully-shown
+    expect(getNextHideSubTasksMode(HideAll, 0, 3, false, false)).toBeUndefined();
+  });
+
+  it('handles a task with no subtasks without stepping into HideDone', () => {
+    expect(getNextHideSubTasksMode(undefined, 0, 0, true, false)).toBe(HideAll);
+  });
+});
diff --git a/src/app/features/tasks/util/get-next-hide-sub-tasks-mode.ts b/src/app/features/tasks/util/get-next-hide-sub-tasks-mode.ts
new file mode 100644
index 0000000000..0c7c57acb9
--- /dev/null
+++ b/src/app/features/tasks/util/get-next-hide-sub-tasks-mode.ts
@@ -0,0 +1,54 @@
+import { HideSubTasksMode } from '../task.model';
+
+/**
+ * Computes the next `_hideSubTasksMode` value when cycling a task's subtask
+ * collapse state.
+ *
+ * Extracted from the old `toggleTaskHideSubTasks` reducer so the *resolved
+ * absolute value* can be persisted via `updateTaskUi` (which is replay-safe)
+ * instead of persisting a relative toggle command. Replaying a relative
+ * command recomputes from live state and is non-deterministic across devices.
+ * See issue #8781.
+ *
+ * @param currentMode current `_hideSubTasksMode` (undefined = fully shown)
+ * @param subTaskDoneCount number of currently-done subtasks
+ * @param subTaskCount total number of subtasks
+ * @param isShowLess step towards hiding more (true) or showing more (false)
+ * @param isEndless wrap around the ends instead of clamping
+ */
+export const getNextHideSubTasksMode = (
+  currentMode: HideSubTasksMode | undefined,
+  subTaskDoneCount: number,
+  subTaskCount: number,
+  isShowLess: boolean,
+  isEndless: boolean,
+): HideSubTasksMode | undefined => {
+  const isDoneTaskCaseNeeded = subTaskDoneCount > 0 && subTaskDoneCount < subTaskCount;
+  // for easier calculations we use 0 instead of undefined for show state
+  const oldVal = currentMode || 0;
+  let newVal: number = isShowLess ? oldVal + 1 : oldVal - 1;
+
+  if (!isDoneTaskCaseNeeded && newVal === 1) {
+    if (isShowLess) {
+      newVal = 2;
+    } else {
+      newVal = 0;
+    }
+  }
+
+  if (isEndless) {
+    if (newVal < 0) {
+      newVal = 2;
+    } else if (newVal > 2) {
+      newVal = 0;
+    }
+  } else {
+    if (newVal < 0) {
+      newVal = 0;
+    } else if (newVal > 2) {
+      newVal = 2;
+    }
+  }
+
+  return (newVal || undefined) as HideSubTasksMode | undefined;
+};
diff --git a/src/app/op-log/core/action-types.enum.spec.ts b/src/app/op-log/core/action-types.enum.spec.ts
index c7880d0796..e157dd2b5c 100644
--- a/src/app/op-log/core/action-types.enum.spec.ts
+++ b/src/app/op-log/core/action-types.enum.spec.ts
@@ -12,8 +12,8 @@ describe('ActionType enum', () => {
   const enumValues = Object.values(ActionType) as string[];
   const mappingKeys = Object.keys(ACTION_TYPE_TO_CODE);
 
-  it('should have exactly 146 members', () => {
-    expect(enumValues.length).toBe(146);
+  it('should have exactly 147 members', () => {
+    expect(enumValues.length).toBe(147);
   });
 
   it('should have 1:1 correspondence with ACTION_TYPE_TO_CODE', () => {
diff --git a/src/app/op-log/core/action-types.enum.ts b/src/app/op-log/core/action-types.enum.ts
index c9dcb680b7..f6f587b0dd 100644
--- a/src/app/op-log/core/action-types.enum.ts
+++ b/src/app/op-log/core/action-types.enum.ts
@@ -76,6 +76,9 @@ export enum ActionType {
   TASK_SHARED_REMOVE_DEADLINE = '[Task Shared] removeDeadline',
   TASK_SHARED_CLEAR_DEADLINE_REMINDER = '[Task Shared] clearDeadlineReminder',
 
+  // Task feature action carrying persisted _hideSubTasksMode (issue #8781)
+  TASK_UPDATE_UI = '[Task] Update Task Ui',
+
   // IssueProvider actions (I)
   ISSUE_PROVIDER_ADD = '[IssueProvider/API] Add IssueProvider',
   ISSUE_PROVIDER_UPDATE = '[IssueProvider/API] Update IssueProvider',
diff --git a/src/app/op-log/persistence/compact/action-type-codes.ts b/src/app/op-log/persistence/compact/action-type-codes.ts
index e0ac03286d..e13adf6033 100644
--- a/src/app/op-log/persistence/compact/action-type-codes.ts
+++ b/src/app/op-log/persistence/compact/action-type-codes.ts
@@ -193,6 +193,7 @@ export const ACTION_TYPE_TO_CODE: Record<ActionType, string> = {
   [ActionType.TASK_REMOVE_TIME_SPENT]: 'TR',
   [ActionType.TASK_ROUND_TIME_SPENT]: 'TRD',
   [ActionType.TASK_ADD_TAGS_SHORT_SYNTAX]: 'TGS',
+  [ActionType.TASK_UPDATE_UI]: 'TUU',
 
   // Plugin actions (U)
   [ActionType.PLUGIN_UPSERT_USER_DATA]: 'UU',