I am not sure I fully understand your description of what you see in YaST > Users and Groups (that is why we love CLI commands and their output in the forums, they show without further explanation how things are)…
But I try to guess what you may have found.
You probably stumbled into the fact that you can not remove the primary group of a user.
In the group definitions you can add users (often an advice given when something “does not work”, most often a bad advice though). But the primary group is coupled to a user in the user definition. It can not be removed because a user must have a primary group. It can be changed to another group, but that must be done in the user definition (I guess the Users tab of the YaST screen).