Known issue with Android 8.0/8.1
Unrelated to Muse, there is a known issue with Bluetooth support on Android 8.0 and 8.1 that causes sporadic disconnects. You can learn more about this issue here. Google is tracking the issue here. This issue is more common in LG phones such as Nexus and HTC/Google phones such as Pixel.
Google has been reported to be actively working on this issue. A fix is expected in the next Android 8 update.
You can connect an Aux cable directly from your phone to your car. Just choose the 'Use phone speaker' option from the Muse app. Disconnect the Muse device from the standard Android settings (not from the Muse app).
If you need help with this issue, just get in touch.