Check your mobo is not shorting on the case in any way. Had an extra standoff screwed in which was shorting against the mobo and causing a restart after shutting down. Also check the back IO shield.
It just doesn't want to be turned off... It wants to live... Must be some new Intel AI system
Is a strange problem though! Maybe it's some strange fault with the motherboard itself...
Try disconnectinh EVERYTHING from the motherboard (including graphics card if you have onboard just for test purposes) - just leave cpu, and memory then try it like that... Have you got another CPU you could test with to try to rule that out?