UIButton title resets to XIB value

Consider this:

class MyViewController : UIViewController {
  @IBOutlet var myButton: UIButton!
  @IBOutlet var myLabel: UILabel!

  override func viewDidLoad () {
    super.viewDidLoad ()
    myButton.setTitle("New button text", forState: [.Normal, .Selected, .Highlighted, .Disabled, .Focused])
    myLabel.text = "New label text"
  }
}

Both myLabel and myButton have placeholder text values in .storyboard file: "Old label text" and "Old button text"

Contrary to my expectation this is what I get when the view appears: "New label text" and "Old button text". Any idea why?

The button is a plain button with plain title (not an attributed title). If it helps, this is happening in iOS Action Extension, when the view controller appears in another app.