Use of ContainerRelativeShape() as .clipShape on iPadOS 15 beta

I'm trying to mask a view displayed inside of the widget to a shape that matches the overall shape on iPadOS 15. If I use it as a .clipShape for my subviews. Here is a simplified example showing what I'm doing. On iPadOS 15, this results in a blue rectangle, rather than a blue round rect as I was hoping for. (The exact same code works fine on iOS 15, and macOS Monterey).

struct InsetView: View {
    var body: some View {
        VStack {
            Text("Top")
            Text("Bottom")
        }
        .frame(maxWidth:.infinity, maxHeight: .infinity)
        .background(Color.blue)
        .padding(10)
        .clipShape(ContainerRelativeShape())
    }
}

Is this expected behavior, a bug or am I just using this API incorrectly? Any pointers would be great appreciated.

Again this is an issue I'm only seeing on iPadOS 15 (betas 1-5, i.e. all betas so far), it works absolutely fine on iOS 15 beta, and macOS Monterey beta.