Don’t think the power supply is resetting (smaller PS) according to scope it drops to around 8v upon that spike. I’ll use my 200a PS and see the waveforms to see if anything changes.
The voltage from the smaller 12v supply was dropping enough for the amp's power supply to restart (start twice).
What if there was a way for pin 13 to start at a much lower voltage than B+, since it does drop to 4v wouldn’t it be best to start it at 4v. I remember when I injected 14v to this pin I was getting 92v rails peak to peak (-/+46v). Perhaps this is what is happening and causing the secondary side to spike. This is just a guess.
That pin does start higher but as the rail voltage reaches its target voltage, the voltage is reduced to where that target voltage isn't exceeded.