I found a random issue that slotGlow was on front of Bouncer sometimes, I did call makeSlot before makeBouncer, no clue what could lead that. Maybe something was executed in background. Does anyone else met this behavior too?

 override func didMove(to view: SKView) {
        let background = SKSpriteNode(imageNamed: "background")
        background.position = CGPoint(x: 512, y: 384)
        background.blendMode = .replace
        background.zPosition = -1
        addChild(background)

        scoreLabel = SKLabelNode(fontNamed: "Chalkduster")
        scoreLabel.text = "Score: 0"
        scoreLabel.horizontalAlignmentMode = .right
        scoreLabel.position = CGPoint(x: 980, y: 700)
        addChild(scoreLabel)

        ballsLabel = SKLabelNode(fontNamed: "Chalkduster")
        ballsLabel.text = "Balls: 5"
        ballsLabel.horizontalAlignmentMode = .right
        ballsLabel.position = CGPoint(x: 780, y: 700)
        addChild(ballsLabel)

        editLabel = SKLabelNode(fontNamed: "Chalkduster")
        editLabel.text = "Edit"
        editLabel.position = CGPoint(x: 80, y: 700)
        addChild(editLabel)

        physicsBody = SKPhysicsBody(edgeLoopFrom: frame)
        physicsWorld.contactDelegate = self

        makeSlot(at: CGPoint(x: 128, y: 0), isGood: true)
        makeSlot(at: CGPoint(x: 384, y: 0), isGood: false)
        makeSlot(at: CGPoint(x: 640, y: 0), isGood: true)
        makeSlot(at: CGPoint(x: 896, y: 0), isGood: false)

        makeBouncer(at: CGPoint(x: 0, y: 0))
        makeBouncer(at: CGPoint(x: 256, y: 0))
        makeBouncer(at: CGPoint(x: 512, y: 0))
        makeBouncer(at: CGPoint(x: 768, y: 0))
        makeBouncer(at: CGPoint(x: 1024, y: 0))
    }

func makeBouncer(at position: CGPoint) {
        let bouncer = SKSpriteNode(imageNamed: "bouncer")
        bouncer.position = position
        bouncer.physicsBody = SKPhysicsBody(circleOfRadius: bouncer.size.width / 2.0)
        bouncer.physicsBody?.isDynamic = false
        addChild(bouncer)
        print("BBB")
    }

    func makeSlot(at position: CGPoint, isGood: Bool) {
        var slotBase: SKSpriteNode
        var slotGlow: SKSpriteNode

        if isGood {
            slotBase = SKSpriteNode(imageNamed: "slotBaseGood")
            slotGlow = SKSpriteNode(imageNamed: "slotGlowGood")
            slotBase.name = "good"
        } else {
            slotBase = SKSpriteNode(imageNamed: "slotBaseBad")
            slotGlow = SKSpriteNode(imageNamed: "slotGlowBad")
            slotBase.name = "bad"
        }

        slotBase.position = position
        slotGlow.position = position

        slotBase.physicsBody = SKPhysicsBody(rectangleOf: slotBase.size)
        slotBase.physicsBody?.isDynamic = false

        let spin = SKAction.rotate(byAngle: .pi, duration: 10)
        let spinForever = SKAction.repeatForever(spin)
        slotGlow.run(spinForever)

        addChild(slotBase)
        addChild(slotGlow)
        print("AAA")
    }