The bands are caused by an interaction between the phone display’s refresh/PWM dimming and the camera’s rolling shutter.

Although the screen’s full-frame refresh is about 60 Hz, the display is not updated all at once. It is scanned line by line, so the effective line-refresh timing is much faster than 60 Hz. At low brightness, PWM dimming also turns the screen on and off for part of each cycle. A rolling-shutter camera records different parts of the frame at slightly different times, so some rows are captured while the display is bright and others while it is dim or off. That produces the dark bands.

The band direction can change with camera orientation because the rolling-shutter readout direction relative to the screen scan changes.

A very fast shutter makes the effect more obvious because it samples only a small part of the display cycle. To reduce banding, use a longer exposure so the camera averages over multiple refresh/PWM cycles, or increase the screen brightness if that reduces PWM dimming.