I have a similar problem ...
I noticed that when there are new posts, the little folder icon is yellow .... but not always.

i.e. ...today when I logged on the Forum, on the main "Forums" page, the folders were yellow for each sub-forum (Survival, Around the Campfire, Long Term)
But when I entered the Long Term Forum, all folders where white, even those threads with new posts; these new posts having white icons in front of them.
But (again...), in the Camp Fire forum, there were yellow folders... with yellow folders in front of the new posts ..

So, it seems to work but not in every forum ....