For the last couple of days I have noticed some odd behaviour with the Notifications page, this is on the website not the app. I'm using Chrome on a Macbook. The page says (e.g.) "There are 6 new posts on a thread you're on...(name of thread)" but when I click the notification to go to the thread there are only 3 new messages and the thread scrolls to a post that is after the first one I haven't read so I have to scroll up a bit to find the first unread one.
I think this is only happening with "threads you're on" not "threads you're watching".