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Putting on my Software engineer cap: In the past I noticed that my previous trucks would play my music on my iPhone at different levels, could be the level it was recorded at or the level last listened. I found it very irritating as I was always adjusting the volume and the truck would not play louder than the iPhone setting for that song.
So either Ford or Apple fixed this by raising volume to max when BT connected...works for me.
Then there's the clever solution Toyota adopted in our 2015 Highlander: The entertainment system remembers the last volume setting for each source. For example, adjusting the FM radio volume doesn't affect the iPod's volume setting.