UIScrollView nested subviews not displayed using Auto Layout - ios

Question

I am trying to generate views displayed in a UIScrollView based on structured (XML) data at runtime. I generate all the views I need and add them to my scrollview afterwards, using Auto Layout to let the scrollview calculate its contentSize. Everything worked fine as long as I was only using UILabels and a custom UIImageView subclass, but now I want to add a nested view containing another view and a label to represent text with a vertical line on the left side.
The general layout code I'm using to add all generated views to the scrollview is as follows:
var previousAnchor = scrollView.topAnchor
views.forEach { (view) in
scrollView.addSubview(view)
view.translatesAutoresizingMaskIntoConstraints = false
view.topAnchor.constraint(equalTo: previousAnchor,
constant: UI.defaultPadding).isActive = true
view.leadingAnchor.constraint(equalTo: self.view.leadingAnchor,
constant: UI.smallPadding).isActive = true
view.trailingAnchor.constraint(equalTo: self.view.trailingAnchor,
constant: -UI.smallPadding).isActive = true
previousAnchor = view.bottomAnchor
}
scrollView.bottomAnchor.constraint(equalTo: previousAnchor,
constant: UI.defaultPadding).isActive = true
And the layout code for my nested subview looks like this (it's a UIView subclass):
addSubview(label)
addSubview(line)
line.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 10).isActive = true
line.topAnchor.constraint(equalTo: topAnchor).isActive = true
line.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
line.widthAnchor.constraint(equalToConstant: 2).isActive = true
line.trailingAnchor.constraint(equalTo: label.leadingAnchor).isActive = true
line.backgroundColor = UIColor.green
label.topAnchor.constraint(equalTo: topAnchor).isActive = true
label.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
label.trailingAnchor.constraint(equalTo: trailingAnchor).isActive = true
label.numberOfLines = 0
label.text = "Here's to the crazy ones. The misfits. The rebels. The troublemakers."
If I use this code, I cannot see this view in my scrollview at all and the scrollview has a warning of ambiguous content size attached to it in the Debug View Hierarchy view. This is somewhat irritating, as using labels without being nested in a view like this does work like a charm. So I started playing around and what really puzzles me is that if I use the following code I can see the view itself (as I gave it a gray background color), but still can't see line and/or label, as both seem have a frame of (0,0,0,0) upon inspection:
addSubview(label)
addSubview(line)
line.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 10).isActive = true
line.topAnchor.constraint(equalTo: topAnchor, constant: 10).isActive = true
line.widthAnchor.constraint(equalToConstant: 20).isActive = true
line.heightAnchor.constraint(equalToConstant: 20).isActive = true
line.trailingAnchor.constraint(equalTo: label.leadingAnchor).isActive = true
line.backgroundColor = UIColor.green
label.centerYAnchor.constraint(equalTo: centerYAnchor).isActive = true
label.trailingAnchor.constraint(equalTo: trailingAnchor).isActive = true
heightAnchor.constraint(equalToConstant: 100).isActive = true
heightAnchor.constraint(equalTo: label.heightAnchor).isActive = true
label.numberOfLines = 0
label.text = "Here's to the crazy ones. The misfits. The rebels. The troublemakers."
What am I missing? I also tried putting all my views into a container view instead of using them as subviews of the scrollview directly, but this did not help with my nested view and brought up other problems.