SwiftUI app broken when upgraded to macOS Big Sur 11.3

I have a SwiftUI app that works perfectly fine on the version prior to the new 11.3 update. The app has a List and on clicking an entry, opens the child view which has a toolbar.

This app has been working fine for months now, and once I upgraded to 11.3, the following happens

• the app opens fine

• it even loads the entries into the list view

• when I click on the entry, the right side is "empty" - no toolbar, no content. But the View is loading! (because the debug statements from the view onAppear are executing)

I have absolutely no idea where to even look. Has something materially changed in SwiftUI or Xcode that would break an app like this?

There are no error messages, warnings etc. - nothing. The child area just stays empty... that's it.

I have a MacBook Pro which I haven't upgraded to 11.3, and the app works just fine there.

I've recompiled with the latest Xcode etc., no use.

Looking for any hints,

thank you,