Strange. But I don't understand why you would want to play over AUX instead of Bluetooth when the A2DP streaming is an available option. That sounds more like a phone setting instead of the headunit. Some phones have the option to output to bluetooth or headphone jack. I know my headunit (aftermarket) is bluetooth, but can't support music streaming, only phone conversations via BT.
Sorry let me clarify on that! I didn't mean to play music through "AUX as in headphone jack". I meant to say I hit the "aux button to scroll through the menu to get to my iPhone as a music source via the apple cord/usb connection.
That being said I have wondered the same thing if its actually my phone that is the issue (maybe the stereo doesn't work in harmony with the latest IOS update etc).
The whole idea to be able to plug in and listen/charge my phone with a DIRECT connection yet have the ability to take calls via bluetooth is very much standard issue with most new cars today.
To recap, if i turn bluetooth OFF on my iPhone, the headunit will play/control music directly via USB connection. Once bluetooth is turned back on with my iPhone, that is no longer possible.