Tried it by command line too. I ran
sudo shutdown -r now
which began properly, eventually reaching
Stopping network time synchronization
Starting...
Done.
then nothing happened. No shutdown. I could keep using my other applications no problem.
THEN I did a shutdown from the Apple menu. Much to my surprise it worked and shut down properly! This is repeatable; I've done it twice now. Weird.
It didn't solve the overall problem though; when I rebooted and tried just Apple-shutdown again without the command-line-shutdown, it hung on the background screen again.
My "workaround", then, will be to command-line-shutdown then Apple-shutdown.