Comment # 9 on bug 855115 from Stakanov Schufter

still the case with Leap 15. 
The users open have to be closed one by one up to the login of sddm with all
users closed. The you have to manually select shutdown. 
This is valid not matter if you have authorized the shutdown by a specific user
of if you are authorizing shutdown by all users in yast. Yast settings are
simply ignored as it seems.