When the most recent poster in a thread happens to be on my ignore list, Discourse won’t mark the thread as completely read for me. I always scroll well past the point I see the ignored poster placeholder, but that thread will show as still having new posts until someone else posts and then I can read past that one.
It does that sometimes. Seems not to do it as often to me of late. Only advice I can think of is to turn off your browser extensions and see if it still does it. If not, it’s one of them. If so, it’s just Discourse.
I’ve found that if I just ignore that thread and don’t open it, after a few days it stops showing up in the “Suggested topics” list.