Comment 8 for bug 1838046

Isn't that the default for all network mounts to hang if they are disconnected.
As they want to preserve data at all cost after the disconnect they'd usually need some sort of forced unmount.

I'd expect it might even have hooks to be ok without the unplugged LAN (or disconnected wireless in other bugs that I've seen).

I'm happy that it doesn't do so automatically, CFIS/Samba would try to reconnect to get things working again.

But for your case you might need to explicitly umount them "opting in to any data loss" via something like:
  umount -f <mp>
or
  umount -a -t cifs -l <mp>

Would adding this before suspend/shutdown help in your case?