EDIT: the problem is solved, it was quite simple. My videos was simply not complying with Apple's standards.
Blank output on video playback
I didn't check Apple's standards for video encoding, it helped a lot: https://developer.apple.com/library/content/documentation/Miscellaneous/Conceptual/iPhoneOSTechOverview/MediaLayer/MediaLayer.html#//apple_ref/doc/uid/TP40007898-CH9-SW4
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