Fold 4 is corrupting videos during recording. It is not visible during the capture only during playback.
I believe I have isolated the problem to the following settings. This increases the likelihood of it occurring but its not 100% so repeat the process a couple of times, you don't need to start a new video. Just zoom in and out while moving around etc and it should happen at least once (You will only see it during playback).
Set capture mode to FHD 60
Video Stabilisation: ON
Auto FPS: ON
Zoom in x3, capture 10 seconds, Zoom back to 1x while also changing lighting environments so the Auto FPS drops from 60 to 30 (Move from well lit to less light), The video stream has potential to be corrupted seconds after this point.
See example here. I am walking holding phone in portrait position. I have about 4 examples now, in others video you can freeze frame and the video footage in completely upside down