I experienced this same thing after the last forum 'upgrade' and it took me months to figure out a fix. I had to go to my forum preferences and delete the values for "threads per page" and "posts per thread" or whatever the options are. Saving the change reset them to their default values (30 for both) and that fixed the screwy last-page-is-not-the-last-page thing. Any choices other than 30 and 30 broke it again.

Unfortunately, that fix doesn't work this time, Pinky and Perky in the control room have broken it more than it was already broken. Am I surprised? Sadly, no.