My workaround (also using encrypted $HOME)
(as root)
mv /bin/umount /bin/umount.d
vi /bin/umount
put in this:
#!/bin/bash
PASS=$*
if [[ ! `/usr/bin/w | grep gnome-session` ]]; then
# need to check for active sessions as su and exiting from shells can call umount
/bin/logout-lightdm # containing suggested scipt pointing to /home/usr/bin/.bash_logout
fi
/bin/umount.d $PASS
exit 0

then
chmod 4755 /bin/umount

Not nice but logout and reboot worked. Or has the issue been fixed?