Why is my App Store preview video only very low quality?

I created an App Store preview video for my app. I used the iMovie template for app previews and exported the video. It looks perfekt when I play the exported video locally. Then I uploaded it to iTunes connect and published my app. In the App Store the app preview video now looks very bad. Kinda like it uses a very low Bitrate and even the colours seem to be reduced. Why does this happen? When I play other videos for other apps on the App Store they look nice on my phone. Except the videos I uploaded for my apps. What am I doing wrong?