Steps to reproduce:

1. Uncheck "Hide categorization of pages" in preferences on Wikipedia (cswiki).
2. Add some (monitoring, maintenance or test) category to your Watchlist.
3. Add sandbox to that category.
4. Edit sandbox one more time.
5. Open your Watchlist.

Expected behavior:
Sandbox should be listed in Watchlist's last changes as added to the category until it is present in that category. Specially for maintenance categories it is useful to know, there was a page added (containing some incorrect wikitext or so).

Current behavior:
On mobile web Wikipedia it behaves as expected, on desktop the sandbox page is in Watchlist's last changes until it is edited. After that it just disappears from the watchlist, but it is still inside of the maintenance category and it just sneaked through watcher's view. The maintenance category is then filled with broken pages, none of which was visible in Watchlist's last changes.