ATX power supply doesn't turn off after PS_ON wire is disconnected from GND
I found the problem, it was a grounding issue. The power supply PCB is supposed to touch the case in the bolting points, but only does so when it is bolted on, making the GND (0v DC) connected to the power plug ground. Since I had been soldering and testing, I had removed the PCB and it happens that I still hadn't put it back on.
Still, a big thank you to those who tried to help!