I'm experiencing this strange issue for some time now (before my 18.04 upgrade).
When I shutdown my computer it reboots right away. I searched the web and tried a lot of suggestions, nothing worked.
I tried sudo shutdown
no difference
I have a dual boot, when I shutdown on Windows, the computer shuts down and doesn't reboot.
I tried sur solution suggested here, I don't have that /etc/grub/module file, and the first part didn't work as well.
I tried to disable WOL as suggested here. Didn't work either.
If you have any extra suggestion, I'll gladly try them.