Custom segue won't resize Subview

Question

I am using a custom segue for transitioning from a collection view cell to a view controller with a kind of zoom in/zoom out style - pretty much like App's do it on the Homescreen, when you open/close them.

In the destination view controller I am adding one of two child view controller. Navigation to the destination view controller works fine, subviews are added and transformed accordingly.

Unwinding the segue however results in a mess. The child view seems to snap behind the actual view and disappears as soon as the actual view's navigation is completed.

Views are added like:

private lazy var documentViewController: DocumentViewController = {
    let storyboard = UIStoryboard(name: "Main", bundle: Bundle.main)
    var viewController = storyboard.instantiateViewController(withIdentifier: "DocumentViewController") as! DocumentViewController
    self.add(asChildViewController: viewController)
    return viewController
}()

override func viewDidLoad() {
    super.viewDidLoad()
    add(asChildViewController: documentViewController)
}

private func add(asChildViewController viewController: UIViewController) {
    // Add Child View Controller
    addChildViewController(viewController)
    // Add Child View as Subview
    view.addSubview(viewController.view)
    view.sendSubview(toBack: viewController.view)
    // Configure Child View
    viewController.view.frame = view.bounds

    // Notify Child View Controller
    viewController.didMove(toParentViewController: self)
}

These are my segues:

func zoom(_ viewCell: UICollectionViewCell) {
    let toViewController = self.destination
    let fromViewController = self.source

    let containerView = fromViewController.view.superview

    let cellContentView = viewCell.contentView
    let rect = cellContentView.convert(cellContentView.frame, to: toViewController.view)
    let originalCenter = CGPoint(x: rect.origin.x + rect.size.width / 2, y: rect.origin.y + rect.size.height / 2)

    let scaleX = rect.size.width / toViewController.view.frame.width
    let scaleY = rect.size.height / toViewController.view.frame.height

    toViewController.view.transform = CGAffineTransform(scaleX: scaleX, y: scaleY)
    toViewController.view.layer.cornerRadius = 50
    toViewController.view.center = originalCenter

    containerView?.addSubview(toViewController.view)

    UIView.animate(withDuration: 10.5, delay: 0, options: .curveEaseInOut, animations: {
        toViewController.view.transform = CGAffineTransform.identity
        toViewController.view.center = fromViewController.view.center
        toViewController.view.layer.cornerRadius = 0
    }, completion: { success in
        fromViewController.present(toViewController, animated: false, completion: nil)
    })
}

func unwindZoom(_ viewCell: UICollectionViewCell) {
    let toViewController = self.destination
    let fromViewController = self.source

    fromViewController.view.superview?.insertSubview(toViewController.view, at: 0)

    UIView.animate(withDuration: 10.5, delay: 0, options: .curveEaseInOut, animations: {
        let cellContentView = viewCell.contentView
        let rect = cellContentView.convert(cellContentView.frame, to: toViewController.view)
        let originalCenter = CGPoint(x: rect.origin.x + rect.size.width / 2, y: rect.origin.y + rect.size.height / 2)

        let scaleX = rect.size.width / toViewController.view.frame.width
        let scaleY = rect.size.height / toViewController.view.frame.height

        fromViewController.view.transform = CGAffineTransform(scaleX: scaleX, y: scaleY)
        fromViewController.view.center = originalCenter
        fromViewController.view.layer.cornerRadius = 50
    }, completion: { success in
        fromViewController.dismiss(animated: false, completion: nil)
    })
}