diff --git a/e2e/tests/planner/planner-subtask-scheduled-visibility.spec.ts b/e2e/tests/planner/planner-subtask-scheduled-visibility.spec.ts
new file mode 100644
index 0000000000..e8af71c94d
--- /dev/null
+++ b/e2e/tests/planner/planner-subtask-scheduled-visibility.spec.ts
@@ -0,0 +1,84 @@
+import type { Locator, Page } from '@playwright/test';
+import { expect, test } from '../../fixtures/test.fixture';
+
+const QUICK_ACCESS_BUTTON = '.quick-access button';
+
+/**
+ * Schedule a task for a quick-access day (day-only, no time) via the keyboard
+ * shortcut ('S' opens the schedule dialog for the focused task) and the
+ * dialog's quick-access buttons (Today=0, Tomorrow=1, Next Week=2, Next
+ * Month=3). onQuickAccessClick auto-submits, so clicking a day both sets the
+ * due day and closes the dialog.
+ *
+ * The keyboard path is used deliberately: it is independent of whether the task
+ * is already scheduled (a scheduled task's detail item shows a "Reschedule"
+ * control instead of "Schedule Task") and of task height (hovering a parent
+ * with sub-tasks lands on a sub-task row).
+ */
+const scheduleTaskForQuickDay = async (
+  page: Page,
+  task: Locator,
+  quickAccessIndex: number,
+): Promise<void> => {
+  await task.scrollIntoViewIfNeeded();
+  await task.focus();
+  await expect(task).toBeFocused();
+
+  await page.keyboard.press('s');
+
+  const scheduleDialog = page.locator('dialog-schedule-task');
+  await expect(scheduleDialog).toBeVisible({ timeout: 10000 });
+  // No time set -> this creates a date-only dueDay, not a timed dueWithTime.
+  await expect(scheduleDialog.locator('input[type="time"]')).toHaveValue('');
+
+  await scheduleDialog.locator(QUICK_ACCESS_BUTTON).nth(quickAccessIndex).click();
+  await scheduleDialog.waitFor({ state: 'hidden', timeout: 10000 });
+};
+
+const getTomorrowDateString = (): string => {
+  const tomorrow = new Date();
+  tomorrow.setDate(tomorrow.getDate() + 1);
+
+  const year = tomorrow.getFullYear();
+  const month = String(tomorrow.getMonth() + 1).padStart(2, '0');
+  const day = String(tomorrow.getDate()).padStart(2, '0');
+  return `${year}-${month}-${day}`;
+};
+
+test.describe('Planner: scheduled subtask visibility (#9019)', () => {
+  test('keeps a date-only subtask visible on its own day after planning its parent', async ({
+    page,
+    plannerPage,
+    taskPage,
+    workViewPage,
+  }) => {
+    await workViewPage.waitForTaskList();
+    await workViewPage.addTask('Parent 9019');
+
+    const parentTask = taskPage.getTaskByText('Parent 9019').first();
+    await expect(parentTask).toBeVisible();
+    await workViewPage.addSubTask(parentTask, 'Sub 9019');
+
+    const subTask = taskPage
+      .getSubTasks(parentTask)
+      .filter({ hasText: 'Sub 9019' })
+      .first();
+    await expect(subTask).toBeVisible();
+
+    // Schedule the SUBTASK for tomorrow (day-only).
+    await scheduleTaskForQuickDay(page, subTask, 1);
+
+    // Plan the PARENT for a different day (next week). Before #9019 this removed
+    // the subtask from EVERY planner day, so it vanished from the planner.
+    await scheduleTaskForQuickDay(page, parentTask, 2);
+
+    await plannerPage.navigateToPlanner();
+    await plannerPage.waitForPlannerView();
+
+    const subTaskDay = page.locator(`planner-day[data-day="${getTomorrowDateString()}"]`);
+    await expect(subTaskDay).toBeVisible({ timeout: 15000 });
+    await expect(
+      subTaskDay.locator('planner-task').filter({ hasText: 'Sub 9019' }),
+    ).toBeVisible({ timeout: 15000 });
+  });
+});
diff --git a/src/app/root-store/meta/task-shared-meta-reducers/planner-shared.reducer.spec.ts b/src/app/root-store/meta/task-shared-meta-reducers/planner-shared.reducer.spec.ts
index 6c9d47bdd9..6497f3f341 100644
--- a/src/app/root-store/meta/task-shared-meta-reducers/planner-shared.reducer.spec.ts
+++ b/src/app/root-store/meta/task-shared-meta-reducers/planner-shared.reducer.spec.ts
@@ -83,6 +83,26 @@ describe('plannerSharedMetaReducer', () => {
       expect(resultState.planner.days['2099-12-31']).toContain('task1');
     });
 
+    it('should not touch a sub-task scheduled for a different day when transferring the parent (#9019)', () => {
+      const todayStr = getDbDateStr();
+      const testState = createStateWithExistingTasks([], [], [], []);
+      testState.planner = {
+        ...testState.planner,
+        days: {
+          '2024-01-16': ['sub1'], // sub scheduled for its own day
+          '2024-01-20': ['other'],
+        },
+      };
+      const task = createMockTask({ id: 'parent', subTaskIds: ['sub1', 'sub2'] });
+      // parent is transferred onto a different day than the sub-task
+      const action = createTransferTaskAction(task, todayStr, 0, '2024-01-20', todayStr);
+
+      metaReducer(testState, action);
+      const resultState = mockReducer.calls.mostRecent().args[0];
+      expect(resultState.planner.days['2024-01-16']).toEqual(['sub1']);
+      expect(resultState.planner.days['2024-01-20']).toEqual(['parent', 'other']);
+    });
+
     it('should update task.dueDay when transferring from today to different day', () => {
       const todayStr = getDbDateStr();
       const newDay = '2026-01-15';
@@ -366,6 +386,27 @@ describe('plannerSharedMetaReducer', () => {
       ]);
     });
 
+    it('should only collapse sub-tasks on the target day, not erase them from other days (#9019)', () => {
+      const testState = createStateWithExistingTasks([], [], [], []);
+      // sub1 is scheduled for a *different* day than the parent is planned for
+      testState.planner = {
+        ...testState.planner,
+        days: {
+          '2024-01-16': ['sub1'],
+          '2024-01-20': ['other'],
+        },
+      };
+      const task = createMockTask({ id: 'parent', subTaskIds: ['sub1', 'sub2'] });
+      const action = createPlanTaskForDayAction(task, '2024-01-20', false);
+
+      metaReducer(testState, action);
+      const resultState = mockReducer.calls.mostRecent().args[0];
+      // sub1 stays on its own day (2024-01-16); the parent lands on 2024-01-20.
+      // Before the fix, planning the parent wiped sub1 from ALL days.
+      expect(resultState.planner.days['2024-01-16']).toEqual(['sub1']);
+      expect(resultState.planner.days['2024-01-20']).toEqual(['other', 'parent']);
+    });
+
     // Virtual tag pattern tests: TODAY_TAG membership is determined by task.dueDay,
     // NOT by task.tagIds. TODAY_TAG should NEVER be in task.tagIds.
     // See: docs/ai/today-tag-architecture.md
diff --git a/src/app/root-store/meta/task-shared-meta-reducers/planner-shared.reducer.ts b/src/app/root-store/meta/task-shared-meta-reducers/planner-shared.reducer.ts
index e58ffb1669..fa5b055cc8 100644
--- a/src/app/root-store/meta/task-shared-meta-reducers/planner-shared.reducer.ts
+++ b/src/app/root-store/meta/task-shared-meta-reducers/planner-shared.reducer.ts
@@ -13,7 +13,7 @@ import {
   getTag,
   hasInvalidTodayTag,
   removeTaskFromPlannerDays,
-  removeTasksFromPlannerDays,
+  removeTasksFromSinglePlannerDay,
   updateTags,
 } from './task-shared-helpers';
 import {
@@ -84,7 +84,8 @@ const handleTransferTask = (
       task.id,
       ...targetDays.slice(targetIndex),
     ])
-      // when moving a parent to the day, remove all sub-tasks
+      // when moving a parent to the day, collapse its sub-tasks under it on
+      // THIS day only (other days keep their own scheduling — see #9019)
       .filter((id: string) => !task.subTaskIds.includes(id));
   }
 
@@ -247,9 +248,12 @@ const handlePlanTaskForDay = (
   // Add to target day if not today (today's ordering is managed by TODAY_TAG.taskIds)
   if (day !== todayStr) {
     state = addTaskToPlannerDay(state, task.id, day, isAddToTop ? 0 : Infinity);
-    // When moving a parent, remove sub-tasks from the target day
+    // Collapse the parent's sub-tasks under it for THIS day only. Previously we
+    // removed them from ALL planner days, which silently erased any day the user
+    // had scheduled a sub-task for once its parent was planned anywhere (#9019).
+    // Matches the target-day-only behaviour of handleTransferTask.
     if (task.subTaskIds.length > 0) {
-      state = removeTasksFromPlannerDays(state, task.subTaskIds);
+      state = removeTasksFromSinglePlannerDay(state, task.subTaskIds, day);
     }
   }
 
diff --git a/src/app/root-store/meta/task-shared-meta-reducers/task-shared-helpers.ts b/src/app/root-store/meta/task-shared-meta-reducers/task-shared-helpers.ts
index eb94f22a79..1183555c9d 100644
--- a/src/app/root-store/meta/task-shared-meta-reducers/task-shared-helpers.ts
+++ b/src/app/root-store/meta/task-shared-meta-reducers/task-shared-helpers.ts
@@ -423,6 +423,42 @@ export const removeTasksFromPlannerDays = (
   };
 };
 
+/**
+ * Removes multiple tasks from a SINGLE planner day, leaving all other days
+ * intact. Distinct from `removeTasksFromPlannerDays` (all days) — the scope
+ * difference is data-loss-sensitive, hence the explicit name.
+ * @param state Root state
+ * @param taskIds Task IDs to remove
+ * @param day Day (date string) to remove them from
+ * @returns Updated state, or original state if no changes
+ */
+export const removeTasksFromSinglePlannerDay = (
+  state: RootState,
+  taskIds: string[],
+  day: string,
+): RootState => {
+  const dayTasks = state.planner?.days?.[day];
+  if (!dayTasks || taskIds.length === 0) {
+    return state;
+  }
+
+  const filtered = removeTasksFromList(dayTasks, taskIds);
+  if (filtered.length === dayTasks.length) {
+    return state;
+  }
+
+  return {
+    ...state,
+    [plannerFeatureKey]: {
+      ...state.planner,
+      days: {
+        ...state.planner.days,
+        [day]: filtered,
+      },
+    },
+  };
+};
+
 /**
  * Adds a task to a specific planner day.
  * Removes the task from all other days first (task can only be in one day).