Ive had the same issue with the volume changing on my iPhone 11 Max Pro in my 2018 Terrain, it was driving me crazy. I searched the web trying to fix this issue. Well Jack you fixed it for me! I disconnected just the Bluetooth connection and only hookup to USB/Carplay and this fixed it for me as well.
I know Apple plays loose with the Bluetooth spec so I believe its there implantation as in the past I had issues with headphones until updates would eventually fix those...