@ChocolateOverflow is your flashlight an LED flashlight? I'm willing to bet it is. What you're seeing is an effect called aliasing, where two or more signals of differing frequencies interact to create new emergent periodic patterns. The rolling shutter effect, moiré, and wagon-wheel effect are all examples of visual aliasing. In your case,

there are 3 frequencies in play: the main light (flickers at 100 or 120 Hz, depending on your location); the flashlight (if it's LED, it is most likely PWM/chopper controlled, probably above 20kHz (so it doesn't audibly hum); and the scan rate of your phone's live view mode. Those 3 frequencies interact to cause visible banding of distinct light and/or dark lines on your screen. When you remove any 1 of the 3 frequencies (either light, or the camera itself), the pattern goes away.