From my limited use so far, if the dash display is set to show maps (versus the basic display without maps), then Google Maps is on the screen unless you have CarPlay connected via wire and are in navigation via Apple Maps. If CarPlay is via the dongle, then Google Maps is replaced with a black display where the map used to be. I haven't tried Waze or Google Maps running via CarPlay yet, though, to see if anything other than Apple Maps projects via CarPlay onto the dash screen. More experimentation is required ;-).I use a wireless adapter for CarPlay which is always plugged in - if you have Google Maps open, does it toggle back to that in the cluster? Or is it always CarPlay if the phone is connected to the wire irrespective of whether you are on Google Maps in the display?
BTW, here is what it looks like with Apple Maps: