That’s usually an issue with the head unit or a weird interaction between the phone and the head unit. My old car used to auto play Music when it connected no matter what the source was on the head unit. My new one is better about not auto playing and respecting the last app.

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.

This is almost certainly due to the car trying to use the old iPod control protocol to tell your device to play. AFAIK that protocol just talks to the Music app, but it's also been obsolete for years.

That's the case for one of my cars, not the other, but (without really knowing) I have a suspicion that it's actually the car doing this. There's a few cues on how its (now outdated) interface seems to want to act like its own full-fledged media playing device, with the phone as merely a backend storage, while my other car is more content to let the phone just act as more of an audio device with a few media controls (pause, skip...).

All of this is speculation.

I deleted the Music app and that stopped it. ️

I have only experienced this when swapping out to youtube or when the application I was playing music in previously gets closed to free up memory after a long period of time in the background idle (you can see this when the application appears to start up from scratch and doesn't resume from its previous position)

Also on iOS 12.1.2. I have never encountered this. I usually play audio in my car with either Pocket Casts or Spotify, and iOS reliably picks up on my last used app. I also have the Apple Music app deleted—maybe that plays into it?

Same here.