The information about adding an image to a category is not always shown in the Watchlist.
An example: the uploader added this file to this category. But although I watch the category, this addition to the category is not displayed in Watchlist changes.
However, adding this file to this category is displayed in my Watchlist:
(diff | hist) . . Category:Novoslobodskaya; 15:45 . . A.Savin (talk | contribs) (File:Метро "Новослободская" - panoramio.jpg added to category)
I suspect that when a category is specified at the moment of a file upload, addition to this category is not shown in the Watchlist. If a category is added later, this information is displayed in the Watchlist.
All category membership and Wikidata changes in recentchanges / watchlist were missing on all wikis in 2018 December (probably completely unrelated to the original report, from April; we'd have noticed if this had been going on for half a year).