Hi magic,
Thank you for your response.
I am aware that the 12v rail feeds 3.3v and 5v, but the 12v rail -in my opinion- isn't supposed to enter a steady decline right after reaching 12v. I'll show you what I mean
When the 3.3v and 5v rails are switched off the 12v rail momentarily jumps back up a bit, but this could be perfectly normal. Btw, the 3.3v and 5v rails are managed by a separate IC and seem to hold their intended voltages perfectly.
My other hypothesis is that the 12v rail is triggering under-voltage protection, but I have no way of proving it.
Regarding the probing, I've taken new measurements by moving the ground probe to the IC receiving the signal, but I don't see much improvement. Do you think all the spikes above 1v are just noise and are probably not triggering OCP?