@Baldie: Are you connecting to the vehicles directly via Bluetooth, or using Android Auto? Furthermore, are you using the stock Phone app, or a third party option? I haven't encountered this issue before, therefore I recommend restarting your phone in Safe Mode next time you're due to establish a Bluetooth connection with a vehicle. This will disable all third party apps, and help to determine if the issue is stemming from an external source. To enter Safe Mode, press and hold the Power button until the 'Power off' prompt appears on the screen then release > Touch and hold Power off until the 'Safe mode' prompt appears then release > To confirm, tap Safe mode.
With Safe Mode enabled, does the issue persist?
To exit Safe Mode, simply restart your phone as you would normally.