为什么UIPickerView将嵌入式UIImageView隐藏在其选择器中？

Question

情景：
显示UIImageView子视图。 Superview是红色的。

问题： UIImageView在选择器中消失。

func pickerView(_ pickerView: UIPickerView,
                viewForRow row: Int,
                forComponent component: Int,
                reusing view: UIView?) -> UIView {

    let isMainPicker = (numberOfPickerComponents == 1)
    let isStartNode = (component == 0)

    var myView:UIView?

    if (isMainPicker) {
       ...
    } else {
        myView = UIView(frame: CGRect(x: 0, y: 0, width: 42, height: 42))
        let imageView = pickerNodeData[row]
        imageView.translatesAutoresizingMaskIntoConstraints = false
        myView!.addSubview(imageView)

        let constraint1 = NSLayoutConstraint.constraints(withVisualFormat: "H:|-0.5-[iv(42)]", metrics: nil, views: ["iv":imageView])
        let constraint2 = NSLayoutConstraint.constraints(withVisualFormat: "V:[iv(42)]", metrics: nil, views: ["iv":imageView])

        myView!.addConstraints(constraint1)
        myView!.addConstraints(constraint2)

        myView!.backgroundColor = UIColor.red
    }
    return myView!
}

这是初始屏幕：

轮换后在这里。
请注意一些UIImageViews [circle（character）]消失：

为什么会这样？
如何让每一行包含其UIImageView？