XF 1.3 Link to first unread is inconsistent


Active member
I seem to be having an issues with the "first unread" link on the thread list page. As I understand it, the thread title should take you to the first unread reply in that thread when clicked. Mine does this at time... and, at other times, it takes you to the first post of the thread, and other times it takes you some place random in the thread.

What could be causing this?