Long story short, the PC was used for gaming and the fans were set to Not spin on the GPU. The PC shut off completely after 10 minutes.
I've now sorted the fans, ran superposition for 10 minutes and the screen went black. PC was still on. Damaged card?
Intel Core i9 9900K HT 5GHz
GIGABYTE Z390 Ultra
AMD ASUS STRIX OC VEGA 64
G-Skill Trident 2x8GB DDR4 4000MHz
Samsung EVO NVM.e 250GB
SeaSonic 1250W Gold
Corsair 900D + Custom Loop.