Seems to be caused by this:
Fast startup is a setting that helps your PC start up faster after shutdown. Windows does this by saving system info to a file upon shutdown. When you start your PC again, Windows uses that system info to resume your PC instead of restarting it.
How wonderful intuitive. They've renamed hibernation and hidden it so we think they have made boot from cold faster...
Instead they've just broken Shut Down so it doesn't do what it says it does on the tin. If I wanted to hibernate I would click the option that says, you know, hibernate.