My iPhone will start auto-playing music over Bluetooth or CarPlay, regardless of the make or model. New BMWs, new GMs, old VWs, new Toyotas, aftermarket units, If I specifically have used Apple Music within the last short bit — recent enough that it’s still in my app switcher and if I switch to it, it doesn’t have to refresh — music will always auto-play on vehicle startup. However, if Apple Music is closed or it’s been long enough that its frozen in memory, it doesn’t.
Regardless of people’s personal experiences, this is the expected behavior of the Music app. If the Music app is active, even if nothing is playing, it’s supposed to start auto-playing music. This has been the expected behavior for years, even before Apple Music existed. I remember my 5 on iOS 6 functioning this way as well.
I don’t use AM very often (I have access since I have Apple One, but I mostly use Spotify) so the “issue” is super rare for me, overall.