The test I did was with the dial back at zero.
That doesn’t remove the dial back electronics from the circuit.

The first time I ran into this was with my old school Snap On dial back light on a motor with a crank trigger.
Revving the motor would just show the timing gradually retard as the rpm went up.
And not like one or two degrees either......... like 10-15.

I knew nothing was physically moving since it was a crank trigger, so it had to be either in the electronics or the light. This was the first time I had ever seen anything that made me suspect the light was reading incorrectly.

I called MSD, they said........ try another light without a dial back.
I said, but I’ve never had an issue with this one....... they said, yeh.... but try another light.
Tried a non-dial back light...... and the timing didn’t move at all.

That led me to trying both lights on several other motors at the dyno, and I never could get it to repeat what it did on the motor with the crank trigger........ but from then on, I knew I couldn’t be “sure” the dial back light was reading correctly....... so I stopped using it on the dyno.

It was a couple years later when I did the test with the light from the guys with the melted pistons.