Other users removed from user groups when a user is updated

Example: UserGrp1 with UserA, UserB, and UserC; UserGrp2 with UserB and UserD. I edit UserB to change/add media. When I save UserB, UserA and UserC have been removed from UserGrp1, and UserD has been removed from UserGrp2. I believe this happens in CUserGroup::updateUsers(),specifically in line 577's DBExecute(), but I haven't debugged it in depth yet. This function is called in users.php line 169.

I mark this as a blocker, because I have had this happen to me, and all of a sudden, without warning, I have entire groups of users who are no longer receiving alerts and/or have no access to their hosts.