Why doesn't propertyChanged fire when the default value is null?

This is a formated version of this question as I cannot edit it

I have this attached behavior:

    public enum TextType { Email, Phone, }
        public static class Validator
        {
               public static readonly BindableProperty TextTypeProperty = BindableProperty.CreateAttached(
            "TextType", typeof(TextType), typeof(Validator), null, propertyChanged: ValidateText);

            public static TextType GetTextType(BindableObject view)
            {
                return (TextType)view.GetValue(TextTypeProperty);
            }

            public static void SetTextType(BindableObject view, TextType textType)
            {
                view.SetValue(TextTypeProperty, textType);
            }
            private static void TextTypeChanged(BindableObject bindable, object oldValue, object newValue)
            {
                var entry = bindable as Entry;
                entry.TextChanged += Entry_TextChanged;
            }

            private static void Entry_TextChanged(object sender, TextChangedEventArgs e)
            {
                var entry = sender as Entry;
                bool isValid = false;
                switch (GetTextType(sender as Entry))
                {
                    case TextType.Email:
                        isValid = e.NewTextValue.Contains("@");
                        break;
                    case TextType.Phone:
                        isValid = Regex.IsMatch(e.NewTextValue, @"^\d+$");
                        break;
                    default:
                        break;
                }

                if (isValid)
                    entry.TextColor = Color.Default;
                else
                    entry.TextColor = Color.Red;
            }
        }

in XAML:

    <Entry beh:Validator.TextType="Email" Placeholder="Validate Email"/>

when I run the application on this page, the TextTypeChanged is not called, unless I change the defaultValue to a member of the TextType enum.