Yeah I've seen that behavior. AFAIK It's been like that since the forum upgrade of a few weeks ago. It happens for every reply that causes a new page (other than the first of course). So the 26th, 51st, etc. reply of a thread will not be immediately apparent from the recent posts list.
It seems to fix itself once someone else makes a reply after yours and given that there have been at least 26 replies in the thread that tends to indicate that the thread is relatively popular and it usually isn't too long before that happens. However even if no one replies after you it does eventually fix itself although I would guestimate that it may take somewhere in the range of half hour to an hour before this happens.
Nice trick of blindly trying the next page URL though. 
On one hand this is likely to be a bug that affects *all* of Stardock's 11 forum sites and therefore is worthy of being reported in Forum Issues. On the other, it's not that dramatic an issue since it does correct itself sooner or later. In any case it's probably a good idea to post this to Forum Issues so that's it's more likely to be seen and addressed by the web team, i.e. Andrew.
