I started having issues today (latest beta 5 days ago) this morning got this, a phone restart resolved it until the next journey and I had the same issue again.
No recent A Auto updates that could cause the issue from today.
Again phone restart was only way to get AA working again.
I have had an issue were Android Auto does not stop when I leave the car, the car emblem remains at the top of the screen until I swipe it away. Maybe related to this?