What phone do you have? Moto X Pure
What plan are you on? 3.0
Does your plan include data, or just talk & text? Data, Talk, & Text
I have been having this issue for awhile. When I have my phone connected to the bluetooth in my car and listing to music using Google Music, the volume decreases whenever I get a notification. It does not come back up until I get another notification. As a point of reference I usually have my car audio at about 20 with the volume on my phone turned all the way up. When it drops after a notification I usually have to bump it up quite a bit for it to be at the same listening level it was, usually 30 or so. And then after the next notification and it “fixes” itself it is blaring loud. It could be my imagination, but it also seems like I really have to crank the volume more to get any noticeable change in volume when it is in the decreased state.