Currently, a user watching a category sees an addition/removal in the category only if the change in the last edit on the article (I assume that the preferences "Hide categorization of pages" and "Expand watchlist to show all changes, not just the most recent" are not checked). This was originally reported on fr.wikipedia.org/wiki/Wikipédia:Le_Bistro.
Steps to reproduce on test.wikipedia.org:
- Watch "Category:Test category"
- Add this category to any page (edit 1) => the addition of the category is visible in the watchlist
- Edit the page again (edit 2) => the addition of the category is not visible anymore.
Expected behavior: editing an article after changing categories should not hide its addition or removal from a category in the watchlist. Instead, not enabling the preference "Expand watchlist to show all changes, not just the most recent" could have the effect of showing only the last change for a each watched category (although I am not sure whether such a restriction is desirable as long as there is no other place to view the history of those changes for a specific category).
The current behavior has the drawback that a significant fraction of additions are never shown, for instance new articles created in multiple edits.