In the last few days I've started to have this same problem.
@SilverStormm, I read your post about updating the unread status in Activity Streams as a potential solution and I tried it, but I'm a little confused because I have never used Activity Streams. Should I only have to update this once and it will apply to all threads, or will I have to do it in each thread? Also, when I click on the Save Changes button it says I must name this new Activity Stream, and I'm unclear if I do this whether it will affect how I normally use the forums. (Apologies if this sounds incredibly stupid, but I use Manage Followed Content exclusively and have never experimented with anything else.)
Dell laptop, Windows 10, Microsoft Edge.