How to make CarPlay and Google maps coexist peacefully
Solution 1:
This is likely an issue with your Starlink media center. Try looking through the settings on it for an autoplay on connect or something similar. In my car's media station, a bluetooth connection will pause and auto-play when using Google Maps, but does not always perform the same when using the USB connection.
Solution 2:
From information I received on Subaru's forum: it apparently is not possible to use Google Maps on CarPlay in the Starlink head unit of the 2018 Subaru Crosstrek Premium, and have it relinquish the audio channel to whatever was using it before it was interrupted for a spoken driving instruction. Only Apple Maps can do that - at least, currently; it's unknown whether this functionality could be added to Google Maps.
Here is a link to the discussion: http://www.subaruxvforum.com/forum/interior-audio/162970-using-google-maps-carplay.html