It's not a mains spike, but an EM discharge caused by the rapid voltage and current changes in the flash, that the parent comment is considering (IIUC).
Yes thankyou. This is what I'm getting at. The momentary power flow in a xenon flash is very large. A great big capacitor charges up and then discharges very rapidly to create the bright flash of light. There is also a high voltage exciter coil that fires to kick the xenon tube into conduction at the beginning of the flash cycle. Both of these could be very disruptive to nearby digital electronics.
I thought the original poster would be in a very good spot to help us determine if the effect if photoelectric or the result of EM because he's got everything set up. All he has to do is repeat the experiment while blocking the light from hitting the Pi.
I'll put it in a box and try in a little bit, sorry have to do a few things first. I'm 99% sure it's light dependent though, I had to aim the flash at the board and keep it somewhat close to get it to lock up (like 6" away or so).
I thought the original poster would be in a very good spot to help us determine if the effect if photoelectric or the result of EM because he's got everything set up. All he has to do is repeat the experiment while blocking the light from hitting the Pi.