Unread threads showing as read

I have started having a problem with some threads that I am interested in not showing as unread and not showing the little bubble with the number of new posts. This is happening with at least two threads for me.

Both threads are bookmarked and in tracking mode. I stumbled across the problem a couple days ago when one of these threads showed near the top of the latest posts list showing with the gray font indicating that I had read it, but I hadn’t seen any new posts on that thread for several days. Yesterday a similar thing happened for another thread. I can find nothing in settings for this but it seems these two threads are going directly to read status even when I should see new posts.