2 things to try:
* disable session management, systemsettings5->startup&shutdown->desktop
session, On Login chose option *other* than "Restore previous session"
That was already set to "Start with an empty session".
* enable logind feature to kill user processes on logout, edit
/etc/systemd/logind.conf , set key
KillUserProcesses=yes
This did not fix it. It's really a bug with sddm.
Thank you,
F