Having the same exact issue. I noticed it would do it more frequently when I had my old Motorola Moto 360v2 paired. But even without it, it still disconnects and reconnects about once every couple minutes.
Model number SM-G960U
Same problem here with intermittent disconnect of my BT on my GS9+ and BMW. Never had this problem with my Note 5 and Android 7.1. What a waste of time and money to pay full price for a phone with reduced capabilities like BT, custom message tones, message led customization, etc. This really stinks.
I found a fix mentioned in a few places to reset all the network connections. This has seemed to help the problem. I am now getting better range on the Bluetooth but it did disconnect once while in my pocket. From what I am seeing this is a known Orea problem.
I tried the network reset; 15 seconds into my 3-minute drive, my car stereo lost the connection and I had to manually reconnect like usual.
Samsung really needs to acknowledge that people are having a problem with Bluetooth, and either fix it, or tell us that there's something buggy in Android Oreo that is causing problems with connectivity and dropouts.