How to center horizontal UIStackView without filling full width?

While setting stackView.distribution = .fillProportionally will give you your desired results, it is probably not the appropriate way to do this.

When 2 or more buttons using the same or almost the same .configuration settings are in a UIStackView, they behave as if stackView.distribution = .fillEqually.

Another approach to get the buttons to behave like other views (labels, "old-style" buttons, etc), is to set configuration.titleLineBreakMode = .byTruncatingTail.

So, use the default distribution (.fill) on your stack view, and add that line to each of your buttons. You may also want to use titleTextAttributesTransformer to set the buttons' custom font:

private lazy var durationButton: UIButton = {
    let button = UIButton()
    var configuration = UIButton.Configuration.plain()
    configuration.title = "AAA"
    configuration.image = UIImage(systemName: "chevron.down")
    configuration.imagePadding = 8
    configuration.baseForegroundColor = .white
    configuration.contentInsets = NSDirectionalEdgeInsets(top: 20, leading: 20, bottom: 20, trailing: 20)
    configuration.titleLineBreakMode = .byTruncatingTail
    configuration.titleTextAttributesTransformer = UIConfigurationTextAttributesTransformer { incoming in
        var updated = incoming
        if let f = UIFont(name: "Inter28pt-Bold", size: 18) {
            updated.font = f
        } else {
            updated.font = UIFont.systemFont(ofSize: 18, weight: .regular)
        }
        return updated
    }
    button.configuration = configuration
    button.translatesAutoresizingMaskIntoConstraints = false
    button.backgroundColor = .black
    button.layer.cornerRadius = 26
    button.clipsToBounds = true
    return button
}()
private lazy var locationButton: UIButton = {
    let button = UIButton()
    var configuration = UIButton.Configuration.plain()
    configuration.title = "BBBBBBBBBBB"
    configuration.image = UIImage(systemName: "arrow.right")
    configuration.imagePadding = 8
    configuration.baseForegroundColor = .white
    configuration.contentInsets = NSDirectionalEdgeInsets(top: 20, leading: 20, bottom: 20, trailing: 20)
    configuration.titleLineBreakMode = .byTruncatingTail
    configuration.titleTextAttributesTransformer = UIConfigurationTextAttributesTransformer { incoming in
        var updated = incoming
        if let f = UIFont(name: "Inter28pt-Bold", size: 18) {
            updated.font = f
        } else {
            updated.font = UIFont.systemFont(ofSize: 18, weight: .regular)
        }
        return updated
    }
    button.configuration = configuration
    button.translatesAutoresizingMaskIntoConstraints = false
    button.backgroundColor = .black
    button.layer.cornerRadius = 26
    button.clipsToBounds = true
    return button
}()
private lazy var actionButtonsStack: UIStackView = {
    let stackView = UIStackView(arrangedSubviews: [durationButton, locationButton])
    stackView.axis = .horizontal
    stackView.spacing = 10
    stackView.alignment = .center
    stackView.translatesAutoresizingMaskIntoConstraints = false
    // don't set distribution ... leave it at the default .fill
    return stackView
}()

Now, using this example viewDidLoad() we get the expected output: