ARSCNView in iOS13 - can no longer create transparent background

in iOS 12, I can set up an ARSCNView as a subview of my main view with the following:

lazy var sceneView: ARSCNView = {

let sceneView = ARSCNView(frame: .zero)



sceneView.delegate = self

sceneView.backgroundColor = .clear

sceneView.scene.background.contents = UIColor.clear



return sceneView

}()


- the result is that sceneView is fully transparent: that is, the contents of the view show through it.


In iOS 13, if I set s sceneView.scene.background.contents = UIColor.red or any other colour, the sceneView show that solid colour, but if I set it to .clear I get a black background. It seems not to be respecting alpha.


Is this a bug, or am I missing something?


thanks

wil