Any update on this issue? I have just got my Nokia 8 and upgraded to 8.1.0 Oreo and... I have the same problem. It works with any headphone BT device like BT headphones, BT speaker but NOT with our cars. Tried with my Honda Civic with the integrated HFT BT 2.0 from 2010 as well as a Toyota Aygo from 2015. Both work fine with old HTC and Samsung Android phones and with iPhone 5 and 6. With this Nokia it connects and then 2s into the audio streaming or voice call the BT service fails and it won't connect anymore.
I have just had to purchase this thing from Amazon that is a BT receiver with embedded mic and handsfree and plugs into my car AUX IN 3.5mm jack. This works fine and it does not disconnect.
I do appreciate that this is not a Nokia issue but related to ALL Android 8.x phones including Google own Pixel phones.