Next time you're in your car let the CarPlay start the music playing. Open the Music app and then swipe from the botton of the phone screen. Just to bring all the apps that are currently open (I think it is called Mutlitask screen?) and then shut down the Music app. The music will stop playing and it will not start automatically next time you connect to the CarPlay. It works for me in two cars (both BMW). The bad thing is that you will need to repeat the same steps every time you play something in the car, otherwise it will start playing automatically next time.
Note: it is not that it is the same song playing all the sime. It is whatever you have in the Play Next list, it will resume playing. But if you select a single song and the Play Next list is empty, it will be the same song that is played again. Hope it makes sense.