SKNode.zPosition causes nodes to flicker by reordering them for 1 frame

When running the sample code below, every 3 seconds the middle sprite is replaced by a new one. When this happens, most of the time a flicker is noticeable. When recording the screen and stepping through the recording frame by frame, I noticed that the flicker is caused by a temporary reordering of the nodes’. Below you find two screenshots of two consecutive frames where the reordering is clearly visible.

This only happens for a SpriteKit scene used as an overlay for a SceneKit scene. Commenting out

buttons.zPosition = 1

or avoiding the fade in/out animations solves the issue.

I have created FB15945016.

import SceneKit
import SpriteKit
 
class GameViewController: NSViewController {
    
    let overlay = SKScene()
    var buttons: SKNode!
    var previousButton: SKSpriteNode!
    var nextButton: SKSpriteNode!
    var pageContainer: SKNode!
    var pageViews = [SKNode]()
    var page = 0
    
    override func viewDidLoad() {
        super.viewDidLoad()
        let scene = SCNScene(named: "art.scnassets/ship.scn")!
        let scnView = self.view as! SCNView
        scnView.scene = scene
        
        overlay.anchorPoint = CGPoint(x: 0.5, y: 0.5)
        scnView.overlaySKScene = overlay
        
        buttons = SKNode()
        buttons.zPosition = 1
        overlay.addChild(buttons)
        
        previousButton = SKSpriteNode(systemImage: "arrow.uturn.backward.circle")
        previousButton.position = CGPoint(x: -100, y: 0)
        buttons.addChild(previousButton)
        
        nextButton = SKSpriteNode(systemImage: "arrow.uturn.forward.circle")
        nextButton.position = CGPoint(x: 100, y: 0)
        buttons.addChild(nextButton)
        
        pageContainer = SKNode()
        pageViews = [SKSpriteNode(systemImage: "square.and.arrow.up"), SKSpriteNode(systemImage: "eraser")]
        overlay.addChild(pageContainer)
        
        setPage(0)
 
        Timer.scheduledTimer(withTimeInterval: 3, repeats: true) { [self] _ in
            setPage((page + 1) % 2)
        }
    }
    
    func setPage(_ page: Int) {
        pageViews[self.page].run(.sequence([
            .fadeOut(withDuration: 0.2),
            .removeFromParent()
        ]), withKey: "fade")
        self.page = page
        let pageView = pageViews[page]
        pageView.alpha = 0
        pageView.run(.fadeIn(withDuration: 0.2), withKey: "fade")
        pageContainer.addChild(pageView)
    }
    
    override func viewDidLayout() {
        overlay.size = view.frame.size
    }
    
}
 
extension SKSpriteNode {
    
    public convenience init(systemImage: String) {
        self.init()
        let width = 100.0
        let image = NSImage(systemSymbolName: systemImage, accessibilityDescription: nil)!.withSymbolConfiguration(.init(hierarchicalColor: NSColor.black))!
        let scale = NSScreen.main!.backingScaleFactor
        image.size = CGSize(width: width * scale, height: width / image.size.width * image.size.height * scale)
        texture = SKTexture(image: image)
        size = CGSize(width: width, height: width / image.size.width * image.size.height)
    }
    
}