Yep, same on different browsers. E.g. Both Safari and Chrome.
I've noticed a new wrinkle, while 'suffering' from this for however many weeks/months it is(!):
It seems to actually be triggered by refreshing
https://forums.macrumors.com/watched/threads . E.g. I may have 2 or no threads listed as unread. If I go back to that page, that's what it shows. But if I then press refresh, the same old thread pops up as unread again. (Also, clicking away from Watched, e.g. on to New Posts, and then back again, does not bring the unwatched thread back; but just clicking on Watched again (without clicking away first) does.)
Not sure if this helps when looking into it? (cc
@arn) - Many thanks!