Same issue here! Using A note 8 and a 2020 subaru forester. I hope someone finds a fix? So far I can only get the call to go through the car if I manually change the source using the android auto phone app (on the car) after the call starts.
Thank you to the google team for tending to this, however I am still experienceing the same issue. I also uninstalled all updates and then when updating the app, it still only updates to the version 2020.05.31.316831277.release. :(.
I am having the same issue on my galaxy S4 tablet. It is quite frustrating since I can no longer use hangouts as it has been integrated into gmail, but requires the newer update :(