unfortunately I don’t have a solution nor a workaround, just a wild guess of reason for this behaviour:
In native calls Bluetooth uses the isochronous profile, i.e. bidirectional audio is transferred with lower bandwidth but less latency to meet real time requirements of a phone call.
I would assume that within AlienDalvik Bluetooth operates in buffered mode to provide ‘normal’ playback mode, which is not real time critical, such as a2dp.
To fix this issue Jolla would probably have to implement switching of Bluetooth profiles from AlienDalvik. I’ve no clue if this is already available, if not how much effort this would be.