There are several threats about this issue, see https://forums.ubports.com/topic/457/the-bluetooth-thread/18 and the files on github (e.g. https://github.com/ubports/ubuntu-touch/issues/766)
Best posts made by Nikfrager
RE: Bluetooth car(Audi RS4) connection doesn't work
RE: "The" Bluetooth Thread
My phone is a bq Aquaris E5HD ubuntu edition and now loaded with UBports image. My car is a Citroen C4 Picasso with the MyWay system from Harman. The version of this system is rneg_D_R50b_R50.03. That should be the newest one.
The behaviour is like this:
First when the two sides dont now eachother and starting the connection/ coupling from the car, all is working. It is possible to make calls and to see the addressbook of the phone.
After this when trying again the connection is not made anymore.
The phone says that the car is connected, but the car says the phone is coupled, but not connected and connection is not possible.