When you use Google Assistant, you trust us with your data and it's our responsibility to protect and respect it.
This may be caused by debris in the small microphone hole often found next to the charging port — have a careful poke around with a safety pin or something similar to clear it, and you'll often solve the problem.
Luckily I have handsfree in my car anyway so I can just go back to that but voice commands are so much easier.
I have today lost the ability to use this with Android Auto again.
My husband had the same issue on his phone newer model than mine but it fixed itself within 2-3 days, so it really confused us why mine was still messed up.
I am getting really annoyed by this.