Because there is a need for a major overhaul of the code base to keep abreast of other IT changes, the code base changes are being phased in over an extended period... and as such, are still being phased in.
Thus, you are going to find quirky problems such as the missing pages will continue until the current version used by PPRuNe is updated.
However, updating is an intensive operation, with much database work required, and I couldn't be sure that the owners of PPRuNe are willing to indulge in the necessary upgrades whilst the forum still works reasonably well.
The recent upgrade to PPRuNe was a "big bang", not really a phased upgrade. There was talk of upgrading straight to 4.x.x, but this was deferred for various reasons - technical, operational and functional.
AFAIK, the pagination issue is simply that ads are tagged to a "hidden" post (or something like that, I wasn't paying attention
), so the last page number is out by one on some threads. However, the last post is indeed the last post, and the page you are directed to is in fact the last page of the thread.
SD