Rebooting instead of Shut Down

[rh71]

Got a new Thermaltake Smart 600W PSU to replace one that I thought was causing freezing issues, etc. Either way, I need a replacement PSU for a future build. But this one has been failing to shut down WIN10, but instead reboots it ever since I put it in. The old PSU didn't do this and nothing else had changed. I tried disabling fast startup which the internet said was a solution but it's not helped. I even installed WIN7 instead (for many reasons) and it's doing the same thing there. Any other suggestions before I return this thing? Not sure what the SMART feature does.

 

[VirtualLarry]

That could be the PSU, could be the motherboard. Is this the old Q6600 rig? If so, and the PSU is new, then I would suspect the Q6600 mobo.