I'm happy to report that the issue seems to have been fixed in iOS 14.5 beta 7!

Oh great! Fingers crossed!

We filed a bug report, but haven't heard back.

Were you able to resolve this issue?

Hi there, Thanks for your response. We've already filed a bug report on 11/13/2020 titled "iOS 14: copying videos using AVExportSession resulting in audio getting out of sync". We'll add a sample app and media file.

Sure, I'll file a bug report. P.S. iMovie and Clips appear to work around this issue. We suspect they're using AVAssetWriter instead of AVExportSession.