This appears to have been solved with one of the recent updates to iOS/Xcode etc. The demo App now behaves as expected in regard to the demo App ("Building a document-based app with SwiftUI") and my own apps.

I reported via Feedback Assistant during the iOS 16 beta and Xcode 14.0 beta. No response. Pls do you have a link to your ticket?

On further exploration - the behavior appears to be systemic to iPadOS 15. Case in point the iOS 15 Mail App. If you open emails by long pressing and dragging to create separate scenes e.g. on the right hand side of the screen, they stack up, as expected. If you try to close one of the emails from the stack (hit "Done") then email is popped from the stack, but either does not close or is restored elsewhere on the stack. Is this an iPadOS 15 bug?