I know exactly what you’re talking about. I see it in the stock mail app all the time. I can reproduce it over and over again. If you have the stock mail app closed completely (closed out of the app switcher/multitasking also), and then open mail and tap on one of your emails to view it, the top of the screen quickly flashes like you said. It’s almost like 3-4 flashes one right after the other REALLY fast. It then does not do this whatsoever unless I close the app completely like stated above and then repeat the process like also stated above. I’ve only noticed this in the stock mail app. I’m certain it’s a software glitch because I have no other issues with the screen at all.