At the bottom of the page is a "Mark all read" option. Once you are satisfied that you are up to date, then click that choice and you should be good going forward.

Also, if you are accessing the forum from different devices (or computers), then occasionally I've see it get out of synch.

It's possible that if you close the internet connection, rather than exiting normally, the forum may consider that you've reviewed all the topics in the forum that you were interested in.