Xamarin Forms - BindableProperty Not Working

We have several BindableProperties in our system. They mostly work, and I haven't come across this problem before. I am testing on UWP, but the problem is probably the same on other platforms.

You can see download this code here to see exactly what I am talking about https://ChristianFindlay@bitbucket.org/ChristianFindlay/xamarin-forms-scratch.git

Here is my code:

public class ExtendedEntry : Entry
{
    public static readonly BindableProperty TestProperty =
      BindableProperty.Create<ExtendedEntry, int>
      (
      p => p.Test,
      0,
      BindingMode.TwoWay,
      propertyChanging: TestChanging
      );

    public int Test
    {
        get
        {
            return (int)GetValue(TestProperty);
        }
        set
        {
            SetValue(TestProperty, value);
        }
    }

    private static void TestChanging(BindableObject bindable, int oldValue, int newValue)
    {
        var ctrl = (ExtendedEntry)bindable;
        ctrl.Test = newValue;
    }
}

This is the XAML:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:TestXamarinForms"
             x:Class="TestXamarinForms.BindablePropertyPage">
    <ContentPage.Content>
        <StackLayout>
            <local:ExtendedEntry Test="1" />
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

I can see that in the setter for Test, 1 is being passed in to SetValue. But, on the next line, I look at GetValue for the property in the watch window, and the value 0. The BindableProperty is not sticking. I've tried instantiating the BindingProperty with a few different Create overloads, but nothing seems to work. What am I doing wrong?