Hi,
Starting from APEX 24.2, it's not possible to reuse existing task detail page in the “Task Details Page Number” item. The following error is displayed:

Is it changed behaviour that wasn't documented or a bug in APEX 24.2? I'm almost sure that it was possible in APEX 24.1.
Steps to reproduce:
- Create TASK A
- Create a new task detail page (in my case, the task detail ID is 2) within TASK A by clicking on the “Create Task Details Page” button
- Create TASK B
- Try to use existing detail page (PAGE_ID = 2) within TASK B and click on “Apply Changes” button.
- The error from the above screenshot is displayed.