I had a situation today where I was under Administration -> Users and clicked on a specific user. Within that user's properties I added another group that they should be a member of. I saved the settings and went back to the user list. I noticed that all other user no longer were part of any groups. I then went into another user and added that same group to their membership which then removed that group from the first user and saved it to the new one.

I am able to work around this issue by instead managing all group membership by going into the group itself and adding users directly instead.