Why does other user stay logged in on seat0 and prevent me to hibernate?

Question

Using openSUSE Leap 15.1 with KDE.

I log in as user1 then switch to user2 and log that on off again. Hibernating as user1 was until recently no problem (if I logged off user2). But now I get this:

$ systemctl hibernate
User user2 is logged in on seat0.
Please retry operation after closing inhibitors and logging out other users.
Alternatively, ignore inhibitors and users with 'systemctl hibernate -i'.

The w- and users-commands show only user1.

$ users
user1 user1
$ w
 10:24:52 up 30 min,  2 users,  load average: 0,32, 0,22, 0,23
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
user1    tty7     :0               09:56   30:37  34.94s  0.03s /bin/sh /usr/bin/startkde
user1    pts/0    :0               10:03    4.00s  0.04s  0.00s w

loginctl shows

$ loginctl
   SESSION        UID USER             SEAT             TTY             
         2       1000 user1            seat0                            
         4       1001 user2            seat0                            
         6       1000 user1            seat0   

Why is that? And how can I get back to the hasslefree previous behavior?

EDIT: I know I can do sudo loginctl kill-session 4. But that is just dealing with a reappearing problem I want to go for good.