Hi.

1. Opt in Timeless.
2. Open some page without categories.
3. Click on wikitext editor.
4. Add manually some category A.
5. Save.
6. The category A missing.
7. Click on wikitext editor.
8. Add manually some category B.
9. Save.
10. The category A appears, the category B missing.
11. Click on wikitext editor.
12. Make a nulledit.
13. Both categories appear.
14. Click on wikitext editor.
15. Remove manually category A.
16. Save.
17. Both categories appear.
18. Click on wikitext editor.
19. Remove manually category B.
20. Save.
21. Category A removed, category B appears.
22. Click on wikitext editor.
23. Make a nulledit.
24. Both categories removed.

It can be a need for phisical extra edit, or a need for rebuild, or waiting for task queue update.
Thank you.
See also T175713, T175714.