I just got my 2019 Silverado LTZ and have been experiencing the SAME thing!.... When the phone connects to the trucks bluetooth, the volume display comes on and actually shows it going down. Then, with each notification I get, the same display comes on and show's how low the volume is. The really aggravating thing about this is that the volume level STAYS low, which has resulted in me missing a lot of phone calls. I just turned off the Change With Buttons setting... and turned phone off and back on... let's see if that does work.