That seems to be the problem. The threads are showing the wrong last page number. It seems that in the break up thread the last page, so far, is really 30 with post number 586, but you only see it going up to page 26 at post 520.What I do is to alter the number of the thread in the address bar. So if it's only displaying 26 pages - but I know there are more - then I change it to 30 and get the extra pages.
Same problem with other threads as well. It would be nice to have a simple way to get to the real last page of a thread.