I have the exact same problem, have had for a while now. No Apple software updates have fixed the issue for a year now. The only thing remedying this is a reboot of both devices after leaving the car: meaning I'll reboot my devices multiple times a day, at the worst. A friend who occasionally borrows the car isn't affected by this. The only major difference with the phone setups is that my hardware mute button is on constantly, as I rely on the Apple Watch to notify on incoming calls. I drive a Mini Countryman SE. Based on earlier readings this is thought to be purely BMW-related, affecting then Minis as well.