I'm sure someone has already mentioned this (and I tried posting on the RPi forum but it's having a 'senior' moment)
This is likely just RF overload.
Stick some foil over the RAM and CPU (and voltage reg) and see if things improve - you mentioned flipping the board over so I would imagine that RF (Radio Frequency) is the culprit (as the xenon flash is basically a wideband (all frequencies) radio transmitter). That close is probably going to couple with some squiggly bit of the silicon and do 'interesting things'
Very interested to see if the foil improves things (alu tape would be a good start)!