Guys, I found the fix. Calling MPMediaLibrary.default().addItem(...) never reaches completion when called on the main thread. However, I wrapped it in a DispatchQueue.global(qos: .utility) {} and this made it work.

@teststation5 check out my last message here https://developer.apple.com/forums/thread/658557?page=2 (not the accepted answer because I'm dumb, didn't know what that button did). Long story short, you gotta wrap the addToLibrary func in a dispatch that's not for the main queue.

@davenorfleet Thanks for the workaround! I'm gonna have to try and tweak these, as it seems that sometimes the playlist fails to add the song (knowing MusicKit, I probably have to add a 0.1 seconds timer before calling the completion). But what's important is that I managed to get past the bug! :D Thanks!

Hello! Any chance you found a solution when playing Apple Music alongside an AVAudioPlayer?

Found the issue. I was initializing the AVAudioRecorder BEFORE switching the AVAudioSession category to .record. At least as of 16.2 beta 4, you now have to initialize it AFTER setting the record category. All fine with me, it's probably the recommended approach, but an error reason would be nice when .record() returns false, I'd been at this for 7 hours total before I finally got it.