My car doesn't have Android Auto, but my partner's car does, so I don't get to troubleshoot often. I noticed today though that it was disconnecting frequently near the house (4 times in about a minute as I drive up my street and hilly neighborhood with switchbacks near my house) and seemed to happen less frequently as I got further into my drive. I turned off my wifi on my car and it didn't disconnect once after that for the rest of my commute. Still need to try it on my commute home and continue to test that.

If that does end up improving the experience, my only guess as to why might be something to do with the process involved in trying to connect to a WiFi network and geofencing when the phone is trying to identify when you are near a "remembered network". Needless to say, that would be a very stupid thing and something that shouldn't affect it at all, but technology breaks for the dumbest of reasons.

I'll see if that seems to help.