Hi, i upgrade from iPhone X (with the bluetooth problem) to the iPhone XS Max and has the same problem.
I reported on the iPhone X to Apple and after several tests and logs sent the engineering department reported that it knows the problem and that it is solving the problem in a future software upgrade. Meanwhile ios 12.1.1 left and the problem persists (on my new iPhone XS Max). Works fine if I switch the Wi-Fi off fully in the settings menu.