After mucking around with all the Plymouth stuff in the earlier comments, I can also confirm that simply unchecking 'Available to all users' on my Ethernet connection in Network Manager fixed the problem of unclean mounts at shutdown and reboot.
After mucking around with all the Plymouth stuff in the earlier comments, I can also confirm that simply unchecking 'Available to all users' on my Ethernet connection in Network Manager fixed the problem of unclean mounts at shutdown and reboot.