It's happening on all forums. Here's a screengrab from when I first started thinking about reporting the issue. You can see that my date of last visit is earlier than the last post on two threads (also affects threads with unread posts).
My high water mark for unread posts seems to be about a day, while I normally visit the site every couple of days.
Very confusing, and still happening - has been a problem for at least two weeks, though at first I thought it was my finger trouble!
All assistance gratefuly accepted.