XF 2.1 After update from XF 1.5 to XF 2.1, first unread post behavior is not as expected

[Amin Sabet]

I upgraded my sites from XF 1.5 to XF 2.1, and members are noticing that clicking previously viewed threads does not properly take them to the first unread post. Is this a known issue with upgrading? Many members have reported this, and I've noticed it as well. Here's an example of a recent member comment:

I have noticed in the Watched Threads, some threads will automatically go to my last viewed post, others go to some random post above the last one posted. Even worse, threads that haven't been active for awhile (I don't know what interval) start at the first post on the first page. Is there a solution for this?