So I still got a problem. If I play it in VLC, it didn’t do anything. Running VLC /path/to/file.mp4 gives this about avformat demux
VLC media player 3.0.8 Vetinari (revision 3.0.8-0-gf350b6b5a7)
[0000000000b68570] main libvlc: Running vlc with the default interface. Use 'cvlc' to use vlc without interface.
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f0a04c0e100] moov atom not found
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f09e0002000] moov atom not found
[00007f09e000d540] avformat demux error: Could not open /path/to/file.mp4: Unknown error 1094995529