I have the same problem, and it’s been happening for weeks, just as you describe.
it happens consistently when returning to the maps app in carplay, while the maps app is providing route guidance to a destination, after switching to a full screen view of any other app.
the good news is that subsequently tapping the map app icon on the left panel will quickly relaunch the app, and automatically restart navigation to the same destination. So the damage has been minimal. It can be a little tricky if this happens right before a complicated intersection that requires taking a turn or exit.
FYI, I often run with split screen view (ie Dashboard) and usually see this problem when returning to that split view from another app. i have no idea if it would happen less if I avoided split screen view entirely. I’m using an iPhone 12 Pro max.
just yesterday I was thinking that I have to see if others are discussing this, so I was happy to notice your post when I got to this forum just now.