I don't know that this is a z issue, it might be my corporate proxy doing something strange. But I just noticed when I click [back] I get the same forum, but if I marked it read I don't see the previous page any more.
ie: The old behavior was that I'd Mark Read, click [back] and I'd still see the thread(s) I just marked. Now, every cycle of [back] and [forward] is refreshing the LRPD.