StackView isHidden attribute not updating as expected

I'm trying to update a UIStackView so that a field displays, should the value of a UITextField equal "Other". Here is my code:

@IBOutlet var stackView: UIStackView!
func updateView() {
    print("UPDATING")
    UIView.animate(withDuration: 0.25, animations: { () -> Void in
         if(self.myTextField.text! == "Other") {
              print("SHOWING")
              self.stackView.arrangedSubviews[3].isHidden = false
         } else {
              print("HIDING")
              self.stackView.arrangedSubviews[3].isHidden = true
         }
         print("Is hidden: \(self.stackView.arrangedSubviews[3].isHidden )")
    })

An example output looks like this:

> UPDATING
> HIDING
> Is hidden: true
> UPDATING
> SHOWING
> Is hidden: true

As you can see, the isHidden attribute is reported as true, regardless of what the code above has set it to. I can't really figure out why that might be, but perhaps someone on here can? Is there something obvious to check? Is there any reason why isHidden can't be updated? (note there are no errors appearing in the output).

Updates on the user interface always have to be done on the main thread (THE LAW).

So wrap you UI updates on the main thead:

@IBOutlet var stackView: UIStackView!
func updateView() {
    print("UPDATING")
    UIView.animate(withDuration: 0.25, animations: { () -> Void in
        DispatchQueue.main.async {  // UI updates on the main thread
            if(self.myTextField.text! == "Other") {
                print("SHOWING")
                self.stackView.arrangedSubviews[3].isHidden = false
             } else {
                print("HIDING")
                self.stackView.arrangedSubviews[3].isHidden = true
             }
             print("Is hidden: \(self.stackView.arrangedSubviews[3].isHidden )")
        }
    })