The bug was found in the latest release (version 1.4.0). The bug can be reproduced on an Android emulator 6.0 and a Google Pixel 3 XL phone.

Correct app functionality

1. start the app, select any activity (e.g., Cinema),
2. select another activity (e.g. Woodworking), and undo it (the current activity return back to Cinema)
3. select a third activity (e.g. Cooking), and undo it (the current activity return back to Cooking)

This undo function works perfect.

Incorrect app functionality

1. start the app, select any activity (e.g., Cinema),
2. undo it (the undo cannot take effect, Cinema is still there).
3. select another activity (e.g., Woodworking),
4. undo it (the undo cannot take effect, Woodworking is still there).

After this, if I select any other activities, and undo them, the UNDO option does not take any effect.

Reproducing video