StackLayout duplicates last one/two children below when layout changes on Android in Forms 1.2/3

I have a StackLayout

        var stack = new StackLayout ();
        stack.Padding = new Thickness (10);

        stack.Children.Add (this.firstTextBox);
        stack.Children.Add (this.spacer1);
        stack.Children.Add (this.secondTextBox);
        stack.Children.Add (this.spacer2);
        stack.Children.Add (this.thirdTextBox);

I call a method asynchronously, when control comes back it updates the text. If the text grows, e.g.

            this.firstTextBox.Text = "First text box First text box First text box First text box First text box First text box First text box First text box First text box ";
            this.secondTextBox.Text = "Second text box Second text box Second text box Second text box Second text box Second text box Second text box ";
            this.thirdTextBox.Text = "Third text box";

Then the display shows the ThirdText box twice on screen, once much lower on the screen.

This happens with Xamarin Forms 1.2 and the pre-release of 1.3 on Android L but does not happen on iOS.