I have had the same problem (or similar - I can't tell for sure by what you are saying).
My problem is this: I am logged in, come to the forum, and hit "new posts" (as I always have). I see a new post I am interested in, click on it, and post in it.
Then when I come back the next time (whether five minutes later, an hour later, or a day later) and click "new posts," that thread is gone.
It is not in new posts, and it is not in the section lower down of posts that are semi-new (forget exact wording). And of course if it's something I was interested in enough to reply to, then I want to see it again
Okay, of course the thread is not really gone, but the only way for me to find it is to do something extra, such as go to my profile and hit my recent posts, or just comb through the various forum sections.
This is completely different behavior from the old forum, which would just turn the post up at, say, the top of the "new posts" (if I had just posted and then hit "new posts").
It has been a bit frustrating (and I wasn't sure if anyone else was experiencing it), as it makes it the hardest to follow the threads that I am most interested in. I had been meaning to post something about it. (And I'm sorry if this is not the same thing you are talking about.)