I'm having the same issue. I Just upgraded to IOS 12.2 Last night and this morning in my truck, it did the same thing again. So the last 2 versions of IOS has now dropped down to 2 bars on my volume. That leads me to believe it's the truck, not the phone or IOS. The only thing I've thought of doing is to turn off Bluetooth on my phone, and just plug it in and use the vehicles apple play. By far the most annoying thing that's happened since buying this vehicle. Side note, We bought a 2019 GMC terrain at the same time. It doesn't do this to my wife's phone or mine. Same 8" screen, hers however has navigation. My truck does not. Ready for them to fix this...