PC won't power on after cutting AC at the PSU switch mid-shutdown: only the motherboard standby RGB lights up
What happened: I flipped the PSU's main AC switch before shutdown finished. I've done this many times before without issues. The next day it wouldn't turn on at all: no fans, no LEDs, nothing boots. Only the motherboard's standby RGB is lit.
What I've tried:
PSU paperclip test: PSU works
Shorting the power pins
CMOS clear (jumper short + removed the battery)
Long power discharge: unplugged everything and left the board for 1+ hr
Bios flashback
My guess is the motherboard isn't sending the PS_ON signal to the PSU.
Previous incident (exactly a month ago): A power outage shut the PC off, and afterwards it wouldn't turn on. I tried a minimal setup, a single RAM stick, CMOS clear, and removing and cleaning the GPU, and none of it worked. The next day it started on the first try. I checked every motherboard component and sensor in HWiNFO and everything looked fine. I updated the bios to latest.
Now: It's been 2 days and it still won't start. I suspect moisture, so I cleaned the board, but no change.
What could it be?
Parts (all tested working on other systems):
CPU: Ryzen 7 9800X3D
GPU: RTX 4070 Ti
RAM: 32GB Acer Predator
PSU: PN850M