What's the issue you're having, exactly?
This:
It seems that something triggered "Add posted-to topics to watchlist" and added every thread you have posted in to your watchlist.
I don't have a clue if that's what's been happening to me. I was questioning in order to understand if you had the same issue as me (an absurd quantity of pages on the Watchlist). From what I've been seeing from other replies, this seems to be the case, but not for the reasons suggested.
Is it still happening? Everything seems normal for my watchlist.
All of the "list unread topics" pages have a tendency to get messed up, though it's often just temporary. I don't know why.
Seems like I unfortunately ended up on the small cases that are not temporary
But it's fine, I eventually started to ignore my Watchlist, I'll wait for the new forum. Marking all as read does nothing. This quite an old bug...