Did you ever get this fixed? I'm having the exact same problem. Mercedes E Class, iPhone X, bluetooth connects and disconnects continuously. I had no problems with my iPhone 6s. I took my phone to the Genius Bar last night and they couldn't find anything wrong with it. I've reset, turned off Bluetooth, restarted phone, re-paired, etc.
Searching online it looks like many others are having this issue. I wish Apple would acknowledge and work on a fix.