Steps to reproduce:
- Start at a page that is on your watchlist.
- Reply to a comment
- Check your watchlist. The page is not listed there any longer.
First seen at huwiki by @Tacsipacsi, who reported it at https://www.mediawiki.org/wiki/Topic:Vh1r8tc7ulwj30zh I have reproduced this. My watchlist prefs settings at huwiki do not automatically add all edited pages to my watchlist.