Same thing happens to me. I thought first it was bad GSM coverage, so I tested with music locally on phone, and with 3 different Bluetooth headsets and the connection breaks frequently with all of them. I agree that Nokia must look into this, hopefully a sw/fw update will fix it
This is a bug with Oreo 8.0, it's fixed in the Oreo 8.1 beta. Unfortunately they have closed the beta at the moment, so hopefully you will find it is fixed in the stable release which is due anytime now.
I have a bad Bluetooth problem since upgrading to Oreo with very heavily distorted audio when receiving calls in my car via Bluetooth. Under Nougat it worked superbly. I have the feeling Oreo 8 has been rushed out and not tested properly, two core functions now don't work on my £500 pound handset,