I’m experiencing the same issue where the phone (iPhone 15 Pro) running iOS 18 Beta is no longer recognised by the car (Range Rover Sport) is no longer recognised as a carplay compatible device. Music etc is only available via Bluetooth. There are no MDM policies or screen time restrictions in place. I have tried removing the phone and car from the car and phone respectively and then repairing but there is no change. This issue first occurred in beta 2 but persists in beta 3.