It seems that viewing new posts resets itself must more often or via a wider variety of events than before.
I find that I can read a couple of messages, then post one or two and new posts appears to work as if I'd logged out and logged back in again. So it means that new posts only shows posts made in the last couple of seconds.
I used the new messages feature to read all posts in the past, so this change in what's happening is driving me crazy.