That's fairly weird... Do you have a multimeter?Ugh... So after a month of waiting I got a replacement X570i from Gigabyte for the one that failed and it won't turn on in my SM580, but will on the cardboard. Has anyone had any grounding issues or know how to fix them? I've never experienced this before in my 20 whatever years of building my computer's.
I'd go for a continuity check from the motherboard stand offs to the ground circuit and +5v / +3.3V pins on the USB header as a start (would indicate a short to ground somewhere, possibly inside the motherboard sandwich), and see if there are any visible clearance issues elsewhere that something live could be hitting the earthed case.
I'm guessing you're powering up on cardboard using the same PSU?
You could also power up on cardboard and check if you're getting a voltage on the case itself to check if the PSU is live, may be receiving voltage from the PSU directly or from a damaged cable somewhere in your cable management.