I did some tests now using the iPhone XS Max camera with following results:
- filming in normal mode -> no flicker
- filming in slo-mo -> flicker for a few seconds, after that no flicker anymore
- filming in slo-mo, portrait -> flicker from left to right
- filming in slo-mo, landscape -> flicker from top to down
if you change the color of the front light and/or the brightness does not affect the flicker speed or amplitude.
As I already wrote, the flicker stops after a few seconds. If you then put a hand in front of the camera while recording and remove it again, it will start to flicker again for a few seconds. After a few seconds, the flicker stops again.
I am not sure if this is PWM or something else that tricks the camera.
I uploaded a video here:
https://youtu.be/TTiTNSmnX4k