I've been having an ongoing problem for months now with long MP4 files exported out of Premiere/Media Encoder playing back out of sync on a Mac with Big Sur in QuickTime/Quick Look. For example, I have a 2:12:00 long MP4 recording of a show. If I suddenly jump from the beginning to the middle, the audio doesn't play at all. It will eventually kick in but it's horribly out of sync. If I start at the beginning and jump forward in shorter segments, it seems to have more luck keeping up but there is still a delay the further I go.
If I play these same files in VLC on the same machine, they work perfectly fine. They play perfectly fine in Premiere. If I load them into Audition, all the audio shows up correctly and plays in sync. If I play them on a El Capitain Mac, they play perfectly fine. On Windows, perfectly fine. If I play them on another Big Sur machine, same thing happens.
I have long MP4 files off an ATEM Mini Pro and they don't seem to exhibit the issue.
I would agree though that this sounds like a Mac thing. I'm actually going back in my archives--I'm an event videographer, so there was a steep dropoff in projects middle of 2020--it seems like this problem began sometime in early 2021. I have tons of MP4 files that were essentially all exported with the same settings. Files before a certain date work fine, afterwards, they have this problem.