Are you accessing it from the middle of a long thread? It may be the case that if you're in a thread, it only looks from that point forward, and you'd have to go back to the beginning of the thread to get the full list.
Thanks, you're right!
And hence... it is a bug (a very minor one)...
As a (common) user will expect that it will present all messages requested at that thread (as perpete wrote)