In Watchlist editor I don't see this topic, it isn't there.
I've done all of these and now on unread topics page it appeared. It shouldn't.
This has been a point of concert for many years.
I ofter reset all the uread threads, hoping all the "unwianted" threads, simply go away because of exhaustion.
You're using the forum the wrong way

Clicking "unwatch" doesn't change what shows up under
Show new replies to your posts. For me, that's a long list of topics I've ever responded to in the past 10 years.
But you can edit your
Watchlist. That's the list that changes when you click "watch" or "unwatch".
Allow
me theymos to blow your mind:
Follow those directions, then just stop clicking "
Show new replies to your posts." and instead click "
Watchlist" from now on.
I know from experience changing this habit is going to take a few miss clicks.