I think I am seeing the same thing. I have created an example project with nothing changed from the template except the body of ContentView is:
GeometryReader { geometry in
RoundedRectangle(cornerRadius: 25.0)
.foregroundColor(.blue)
.frame(width: 100, height: 100)
}
.background(Color.green)
I am using a phone running iOS 14 beta 2.
If I build and run from Xcode 11.5 I get a blue rounded rect in the centre of a green background. If I build and run from Xcode 12 beta I get a blue rounded rect in the top left corner of a green background.
As this is labelled as a 'Resolved Issue' in the release notes linked above, does that mean Apple thinks it is working as expected and will not change?