Had the same issue, delete the app data on your phone, re-launch the app then connect to the car, should be visible again. The issue is fixed in android 8.1, its to do with the versions of AVRCP implemented in Android 8.0, or more likely the lack off, AVRCP version 1.3 will work perfectly in this car but it was removed, Android 8.1 adds it back in. You can check this via Developer options on your phone but in the meantime, deleting the app data on your phone will kind of work.
@Carl1983 what version is your firmware after the upgrade from lexus?