On one of my test systems, I have /usr on a separate partition. When I go to reboot this system with systemd-11-1, /usr fails to unmount: /umount: /usr/: device is busy and then reboot.service enters failed state (and the system still does not reboot).
This has been fixed now in v13. The earlier umount might continue to fail (but probably even won't), but this won't stop the system from going down anymore. Also, right before halting the machine we try harder to completely unmount everything that might be remaining because it failed due to busy earlier. I think this resolves this issue comprehensively.
Verified fixed. Thank you!