Comment # 5 on bug 1209824 from Thorsten Kukuk

(In reply to Andreas Vetter from comment #3)
> (In reply to Thorsten Kukuk from comment #2)
> > (In reply to Andreas Vetter from comment #1)
> > > Two ways come to my mind to solve this:
> > > 1) Set KillMode=control-group for rebootmgr.service
> > 
> > This is the default...
> 
> Aehm, not in this case:
> schnuffi:~ # systemctl show rebootmgr.service | grep KillMode
> KillMode=none

That bug got fixed 2 years ago. You need rebootmgr 1.3 or newer. Looks like
Maintenance did not release it for SLES/Leap and I don't know why it was not
released via SLE Micro -> Leap Micro for Leap.
This is a maintenance problem, nothing I can fix.