Why does Car Play disconnect when I go in/out of WiFi range?
I finally found a solution to this. I removed the CloudFlare 1.1.1.1 app from my phone. 

That app provides fast, secure DNS connections. It looks like a VPN profile to your phone, although it is not actually a VPN. Apparently 1.1.1.1 (and maybe some other VPN apps) are breaking CarPlay. I don't know why there is an interaction between networking and CarPlay, but in my before and after tests, it was rock solid once I got rid of the CloudFlare app. 

The DNS app seemed like a good idea, but it was not worth all the trouble it caused.