Using FF.I just tried it on my system, and the 'MARK READ' function/button works properly, imo. It did not take much time for the changes to reflect on my Forum account (the red color icon changes to grey). Which web browser are you using ?
Maybe try clearing the cache/cookies of your web browser ?
At the risk of speaking too soon.When I finish reading a forum I click on the mark read button.
This seems to take much longer to complete than in the past.
It does not matter which forum.
Maybe I'm just imagining things.