It tried to turn on the main DC PSU and I got a short circuit. After a bit more testing, I realised there's a dead short through the two transistors that pull the DC PSU enable line up to VDC when the STM32 output goes to 3.3V(VSB).
I added a resistor and tried to jump 3.3V into various points to test it in stages... and it went pop and now always short circuits, even with the transistors removed. I fear I put 12V into the STM32 and blew it up. Oh no!