Thanks Martin!
It never happened with my old phone (Samsung) and I have never seen this before, which is why I thought it might be Fairphone related.
I tried to troubleshoot with no success :
I wanted to upload a video from my laptop to see if it would still happen (you can emulate an iphone from Safari) but then I could not upload the video at all (MP4).
I also used InShot to re-export it (not sure if the encoding would have changed?) but it still did the same.
It doesn’t happen at the preview stage, only after the upload.
The thing is I use it for personal reason only, it is not a work tool so I am not that bothered to spend hours looking into it, but if I’m not alone having this issue and if there is a quick fix, I’ll happily take it. If not… it’ll just have to be my signature I guess 