Swift - Adding labels to stack views programmatically

My goal is to add labels to a stackview in a xib. Everything below works great if the results are being displayed using a view controller. But I don't need to display the view I'm creating. I just want to add it to an array. (The array will be converted to a PDF later.) For some reason, if I just save the view, anything in the stack view will not appear when the PDF is created. My actual xib has many other fields that show up just fine. It's only the fields I try to programmatically add as labels to a stack view that fail to appear!

This is my xib. The stack view is pinned to the edges and it's set to fill equally.

And this is the code for the xib. There's one outlet for the view and one for the stack view.

import UIKit

class ContainerView: UIView {

    @IBOutlet var contentView: UIView!
    @IBOutlet weak var testStackView: UIStackView!

    override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setup()
    }

    func setup() {
        Bundle.main.loadNibNamed("ContainerView", owner: self, options: nil)
        addSubview(contentView)
        contentView.frame = self.bounds
    }
}

And finally, here is the class that adds labels to the stack view. It has been simplified considerably.

import UIKit

class PreparePDF : UIView {

    func addMyArrangedSubviews (
        let containerView = ContainerView()

        let testLabel1 = UILabel()
        testLabel1.text = "Hola"
        testLabel1.backgroundColor = .red

        let testLabel2 = UILabel()
        testLabel2.text = "Hello"
        testLabel2.backgroundColor = .yellow

        containerView.testStackView.addArrangedSubview(testLabel1)
        containerView.testStackView.addArrangedSubview(testLabel2)  

        pdfPages.append(containerView)
}

I've tried to ask this question several times, but I think I was making it sound too complicated. I'm not sure this is any better!