Wirelessly posted (8820)
If you are sync'ing with outlook or something, I wonder if the contact is being changed somehow, even if just over-written by Outlook (or whatver) such that the blackberry sees it as changed, and this causes the blackberry to drop it from the group. I agree this wouldn't be sat if that is what is happening. I'm just trying to speculate on ideas that might explain what's happening that you wouldn't notice.
Perhaps you could test it by editing something in, say, the street address, on the computer, then sync, and see if that contact gets dropped from the group.
Just grasping for ideas here. As I said above, I've used groups, but never had this problem.