No it’s not that. Trust me when you see the issue it’s so juddery like it’s going at 10 FPS in slow motion.
If you are right, how come it’s only video in 24fps that stutters, please check out this 1080p60fps HDR video. Please open in Youtube app to see it in 60fps HDR.
Here you will see no stutter, and video recording in 1080p60fps on you iPhone also is perfect. This is sadly the downside of OLED. The only fix would be Apple implementing some kind of trumotion like on LG tv’s