When you turned the computer off 'at the wall' during diagnostics was this also powering down the display that had the through-connected DisplayPort pin-20 cable?

So when the rig was plugged back in both the display and the computer were powered up together?

Interesting that the video card implementation does not protect itself from an out of spec DisplayPort cable. An isolated/separate 3.3 volt feed to the DisplayPort cable would seem prudent. Running an internal system power bus to anywhere outside the computer case always feels risky to me.