Can anyone tell me why this code crashes on iOS 15 when Approachable Concurrency is enabled?

```
struct MyModifier: ViewModifier {
var perform: () async -> Void

func body(content: Content) -> some View {
content
}
}

struct ContentView: View {
var body: some View {
Text("Hello, world!")
.modifier(MyModifier(perform: {}))
}
}
```

Marking the perform closure in MyModifier as concurrent or MainActor stops it crashing.