I tested it on a few other threads on the site and it seems to be working fine now including this thread. It happened on two separate threads consecutively.
If it acts up again I will make note of it here.
You go to the last read post in the thread only when you are in linear mode, but when you are in threaded mode there is no way: you are redirected to the starting post no matter what was your last read.
And it only happens when you have DBSEO installed. I never found this issue before and there isn't in fora without it.