Silverblue 33 Failed unmounting /usr on shutdown/reboot

I have this error for quite a while on Silverblue 33. Is this a known bug?
The same error targeting /var vanished some time ago.

journalctl -r -b -1 --system
Isnt much use.

user systemd[1]: Failed unmounting /usr.
user systemd[1]: usr.mount: Mount process exited, code=exited, status=32/n/a
umount[5602]: umount: /usr: das Ziel wird gerade benutzt.

Anyone??