Reason I said that was that this happened same same on the last upgrade. To be sure I don't really know if it's a bbs, apache setting, or some special page coding.

I've seen the same on other bbs using this software. Initially the back button provides a refreshed page, then the admin tosses some switch and pages returned to, via the back button, have to be explictly refreshed.

Edit: Whatever the setting, using the links at the top or bottom of the page generally result in a refreshed view of threads.