I have a Note 4 which worked fine for the last 12 months with native rSAP, but since Marshmallow, it disconnects from the car after a few minutes into any call.
Sometimes it drops the connection after 1 minute, sometimes it's 60 minutes or more, but drop the connection it will.
When I try to re-connect, it usually fails until I manually unlock the phone (sometimes the phone has reset itself, other times it just sits there thinking it is still connected to the car (the Bluetooth page shows that it's connected).
The network sent me a new handset, which was preloaded with 6.0.1, and the problem is identical.
I wondered if your app might help, but it tells me that the phone is not rooted, and I don't know how to do this.