I have been using android auto using car display for more than a year on nexus 5. No problems at all, well except from huge battery drain on Nexus.
Couple of weeks ago updated the phone to Samsung Galaxy s8. Android auto connects and works except for the one part. If I get any notification e.g. on WhatsApp, phone goes in non stop notifications tone. By that I mean, whatever sound phone plays for WhatsApp message notification keeps on playing. I have tried disconnecting phone from android auto, killing all applications. Nothing works. Only recourse is to reboot the phone.
This only happens when phone is connected to android auto display in the car. Otherwise notifications are normal.
On the lighter note, first time this happened, it was almost comic for me. By sheer coincidence I had low fuel signal and WhatsApp message within seconds of each other. Unfortunately, the tone setup for WhatsApp is very similar to car warning. I thought my car has developed some sort of issue and warning me about it and decided to pull over. Only after killing the engine, I discovered that noise was not stopping and found the phone was the actual culprit.