Updated to 12.1.4 today, started listening to an audiobook in the Audible app, paused it, went to home screen & locked phone, got in my car, plugged in USB/lightning cable, started car, audiobook automatically began playing.

I have had it default to Apple Music when doing the same thing as above but with the YouTube app. I think it has something to do with YT not allowing background audio playback unless you sub to Premium. Which reminds me how much I miss Jasmine and ProTube.