This problem happens on my Ubuntu system as well. When it does, log in again, open a terminal, and run
$ ps aux | grep suspend
If you have the same problem I do, you will see a failed/hanging suspend process. That's what causes the system to start right again, because the suspend isn't finishing itself.
If in the same terminal you try this
$ sudo /sbin/shutdown -h now
you will get a refusal from the system, with a message saying something like that's a destructive transaction, that's the system d's way of saying that as long as that suspend process is hung, you aren't shutting down.
The only fix I know of is to kill the suspend process
$ kill -9 nnnnnn
nnnnnn is the number of the hanging suspend process you see in the
ps aux output.
I tried to track discussion of this in forums on the kernel and drivers, but it is pretty tough going. Some people claim it is being solved by kernel updates, say into the latest
4.8 editions, but I don't see any benefit. My problem, I'm pretty sure, links back to commercial Nvidia drivers that don't always get out of the way.