Why is the content of my UIScrollView scrolling behind my UIStackView? - swift

Question

I made a StackView on top of my ScrollView. The bottom constraint of my StackView is constrained to the top constraint of the ScrollView. I use the ScrollView as a container to display my content. The StackView consists of 6 buttons and serves as a navigation. Hierarchy looks like this.
To add a new controller into my ScrollView I use the following code:
#IBAction func planAction(_ sender: UIButton) {
let newPlan = sender.frame.origin.x
scrollView.subviews.forEach{$0.removeFromSuperview()}
let storyBoard = UIStoryboard(name: "Main", bundle: nil)
let nextController = storyBoard.instantiateViewController(withIdentifier: "PlansViewController2") as! PlansViewController2
addChildView(viewController: nextController, in: scrollView)
self.movingView.frame.origin.x = newPlan
}
public func addChildView(viewController: UIViewController, in containerView: UIView) {
viewController.view.autoresizingMask = [.flexibleHeight, .flexibleWidth]
viewController.view.frame = containerView.bounds
addChildViewController(viewController)
containerView.addSubview(viewController.view)
viewController.didMove(toParentViewController: self)
}
In my PlansViewController I add a UIBarButtonItem programmatically but it is not displayed. Look here.
public override func viewDidLoad() {
self.navigationController?.setNavigationBarHidden(false, animated: false)
if let viewControllers = self.navigationController?.viewControllers {
if !viewControllers.contains(where: {
return $0 is StructureNavigationListViewController }) {
let filterBtn = UIBarButtonItem(title: "Kategorienliste", style: .plain, target: self, action: #selector(handleCategoryListTap(sender:)))
navigationItem.rightBarButtonItems = [filterBtn]
}
}
Is my hierarchy wrong or what´s going on here?