I found an old topic and posted this yesterday but I can't see my post anywhere to starting again for awareness. I was asked by google support to post my issue on here. Link to video of the issue is below:

Error message: if available  

Phone make & model: Google Pixel

Android OS version: 9 March 5th Security

Android Auto app version: 4.0.590434

Google Play services version: 13.9.17-all

Google app version: 9.31.8.21.arm64

Below is the recent explaination I sent to Google support:

Hi Samuel,

Apologies for the delay but here is a video if the issue. I believe many other people are experiencing the same thing. Have spoken to colleagues etc who confirm same issue. Just to be clear, I am using the Android Auto App on my google pixel 2, connected to my car through the bluetooth audio connection and also phone capability.

The issue clearly comes to the fact that enabling the google assistant pauses the audio which in turn means the car head unit mutes audio to allow the phone call channel through (car thinks I am receiving a phone call due to the audio being paused). Unfortunately it seems the google assistant voice is routed through media audio (which has been muted due to the song being paused), instead it should be routed through the same audio channel as a phone call.

Step by step:
1. Android Auto app is playing spotify audio through bluetooth to cars head unit
2. I enable google assistant by pressing the mic button on android auto app
3. I speak my command / request
4. Google assistant starts to respond but by which time the head unit has muted the audio for music as it thinks I am receiving a phone call
5. Google assistants response is then muted until it finishes speaking
6. Spotify audio then starts playing again and car head unit un mutes audio

The google assistant needs to be routed as if it is a phone call. This way when the car head unit mutes the audio (music) I will still hear the assistants response.

Cheers