If iOS 12 doesn't play the first song that I ever downloaded from iTunes every time I start my car, then it's a roaring success.
(I know every noise of Kayne West's "Can't Tell Me Nothing" by heart)
This is not an iOS bug, really. It's not iOS playing those songs, it's your car.
The car is enumerating the complete list of songs on your device (using some old iPod-era USB protocol) and auto-playing them itself. It really should be remembering your last played song and picking up from there, or simply give you an option to disable auto-play.
I don't really see how this is something Apple can fix, unfortunately.