I experienced this myself on any XenForo board I visit.
Known Bug on XF Community:
Something related to the recent update of Chrome / conflict on the BBCode editor.
All I can do is to...
If you just view the unread alerts on this pop-up menu:
The thread will remain unread to you and XenForo will stop alerting you in the future post replies.
You can view Unread Watched Threads here: https://hardforum.com/watched/threads
Or if you just open the thread itself, but stops...