Idk why I'm having trouble posting this unless the forms censor out brand names, but all tests were run on my i phone X S running i os 12 and in my Je ep Gran d Cherokee running J eep's uconnect software.
I've been searching for MONTHS trying to even find someone else with teh same issue. I can confirm i have this exact same issue in my cars with iOS12. I noticed it as soon as i tried out the betas back in the summer.
My issue does not occur when using the USB cable in my BMWs. But with Bluetooth the issue is there. I'm going to try turning off the wifi as i've seen suggested previously.
With the 12.0.1 update I can't listen to music through Bluetooth at all, tried re-pairing multiple times and did a fresh install but still the same.
I can call through Bluetooth in the car but when I go to Media it says "Not connected" so the new update killed music playback through Bluetooth all together for me.
EDIT: Put the Xs Max in DFU and installed the firmware again from iTunes, did a complete restore from iCloud and was able to restore Bluetooth again but the issue still remains with the 12.0.1 update. I can confirm that disabling the Wifi from Settings does resolve the issue for me!