Yes, I think I recall you posting that before too. The group set up I gave above was just to give you a clear example of max post count and the user being dropped from that group. We've actually got groups set up with 3 - 5 posts and 6 - 8 posts - they're both quite large groups which shouldn't really make a difference but those are the 2 groups that seem to be dropping users the most noticeably.
I've been keeping an eye on it for a few days, it's only when they log out then log back in that the problem seems to arise. But it's unlikely to be happening to everyone in that scenario or I'd really know about it!