I have a problem where some mp4 videos (either local or url) show a black image and just play the sound on iOS 11. On iOS 12, I do not let me play video (the game button has a dash over that). The simulator (for both of us) plays well.
After much digging, I managed to understand that those who do not play properly are those who have an interlaced scan type. The progressive is okay. (I put back and forth video coding, from those who worked well and those who did not, and this confirmed my theory).
Having said that:
Any idea why this happens?  Can I in any way configure AVPlayer to properly play these files?