Can't play streamed music and navigation at the same time over bluetooth

[AC Question]

I have a HTC 10 and successfully connected via Bluetooth to my Alpine car stereo.
The streamed music from Qobuz plays fine but when I open maps it mutes the music to play the navigation but no sound comes through the stereo, then the music continues. if I switch the music off, navigation on its own comes through fine. I've tried various different settings with no luck. It worked fine with my Huawei P8 using all the same android settings. Ideas?