It looks like both in the final project and my own, adding a NavigationView to the body of ContentView causes it to disappear from the screen. ViewDebugger seems to indicate that a UIHostingController exists, but nothing is being drawn.
Edit: It looks like it runs fine in Canvas, but not on simulator. And by adding a Text view outside of the VStack, I can get the screen to appear on the simulator (w/o the title and a non-functioning “play bullseye” button), but it also breaks the appearance in the Canvas. I’m assuming this is a (bunch of) SwiftUI bug(s).