Figured out that a utility from the motherboard manufacturer was causing this issue. It's the GIGABYTE On/Off Utility. Uninstalling this fixed it, the PC shuts down properly now.

I am in a similar situation. Upgraded a self build PC from Windows 7 to 10. Now, I am unable to shutdown. I have turned off the Fast Startup, but it still reboots when I hit 'Shutdown'. Appreciate...