Sorry missed this, let me see why that is.
What browser and OS are you using? Probably a browser issue and/or a cookie issue on the forum.
EDIT: Got some info on this issue after a short research. Right now the forum is cookie based instead of database based in tracking read/unread threads. Tracking via the cookie method uses the least amount of server resource than it is for the database method, except it is dependent on your inactivity value (15min) rather than the last login time. Some browsers may limit the cookie directory size and that will have an affect on keeping account of your read/unread threads.
We can go to the database method seeing that the last time we tried it we were having server problems unrelated to this issue. Some people like the cookie method and some don't, so we can try the database method and it will be set to 10 days. Any thread older than 10 days will be considered read. If you think we should change the day value, we can do that.
After changing it to the database method, the format will look a little different and since the database tracking has just been turned on, a lot of the threads that were once read are unread again. You can click on this link...
https://sprinter-source.com/forums/forumdisplay.php?do=markread
...to cause everthing to go read.