SKRenderer not rendering alpha channel of SKShapeNode in MTKView

Question

I've set up an SKRenderer to render an SKScene into an MTKView on macOS. However any SKShapeNodes in the scene render like this as though they're ignoring the alpha values:

Here is my ViewController where the SKRenderer is set up:

import Cocoa
import SpriteKit
import MetalKit

class ViewController: NSViewController {

    var device: MTLDevice!
    var renderer: SKRenderer!
    var commandQueue: MTLCommandQueue!
    var scene: GameScene!
    var mtkView = MTKView()

    override func viewDidAppear() {
        super.viewDidAppear()

        device = MTLCreateSystemDefaultDevice()
        renderer = SKRenderer(device: device)
        commandQueue = device.makeCommandQueue()

        scene = GameScene(size: view.bounds.size)
        scene.scaleMode = .aspectFill

        renderer.scene = scene

        mtkView.device = device
        mtkView.delegate = self
        mtkView.isPaused = false
        view.addSubview(mtkView)
    }

    override func viewDidLayout() {
        mtkView.frame = view.bounds
    }

}

My MTKViewDelegate where the SKRenderer renders in Metal:

extension ViewController: MTKViewDelegate {

    func mtkView(_ view: MTKView, drawableSizeWillChange size: CGSize) { }

    func draw(in view: MTKView) {
        let commandBuffer = commandQueue.makeCommandBuffer()!
        let renderPassDescriptor = view.currentRenderPassDescriptor!

        renderer.render(withViewport: view.bounds, commandBuffer: commandBuffer, renderPassDescriptor: renderPassDescriptor)

        commandBuffer.present(view.currentDrawable!)
        commandBuffer.commit()
    }

}

And my basic GameScene:

import SpriteKit

class GameScene: SKScene {

    override init(size: CGSize) {
        super.init(size: size)

        anchorPoint = CGPoint(x: 0.5, y: 0.5)
        let circle = SKShapeNode(ellipseOf: CGSize(width: size.width / 2, height: size.height / 2))
        circle.fillColor = SKColor.green
        circle.strokeColor = SKColor.blue
        addChild(circle)
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)

        anchorPoint = CGPoint(x: 0.5, y: 0.5)
        let circle = SKShapeNode(ellipseOf: CGSize(width: size.width / 2, height: size.height / 2))
        circle.fillColor = SKColor.green
        circle.strokeColor = SKColor.clear
        addChild(circle)
    }
}

I'm pretty new to Metal so I'm sure that I'm just missing something basic.