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Best guess is that the low latency video player doesn’t use AVPlayerItem. This is a piece of code iOS provides apps to play video so each app doesn’t have to reinvent video playback from the ground up. It’s quite useful if you’re an app developer who wants to add video. However, it comes expectations around video formats. I imagine low latency mode doesn’t have enough of a buffer to send to AVPlayerItem to make it work and has to do something custom in the app to play video.
That said, it’s been three years since I’ve had to do iOS development so I could be off on the details here.