The #Apple QuickTime Player, and thus the entire #macOS AVFoundation framework have massive problems with some h.265 video codec files.
They can only play the "hvc1" variant of this highly efficient video codec, but not the "hev1" version.
Sigh.
So the integrated Movie Converter in my #NeoFinder 8.9.1 now makes sure to generate the proper h.265 variant that is understood by Apple and macOS.
https://www.neofinder.de/forum/phpBB3/viewtopic.php?p=667#p667