`Scope.Errors` incorrect when set one invalid `TextBox` invisible

[mzh3511]

Hello, I found that Scope.Errors is incorrect when setting one invalid TextBox invisible if there are more than 1 invalid TextBox

1. Add a button to visible or invisible DoubleTextBox in the Gu.Wpf.ValidationScope.Demo.OneLevelScopeView, the button event method code:

    private void BtnEnable_OnClick(object sender, RoutedEventArgs e)
    {
        if (DoubleTextBox.Visibility == Visibility.Visible)
            DoubleTextBox.Visibility = Visibility.Collapsed;
        else
            DoubleTextBox.Visibility = Visibility.Visible;
    }

2. Input English letters in both IntTextBox2 and DoubleTextBox
   Expect: Get 2 errors, both IntTextBox2 and DoubleTextBox have the red box
   Actual: Same with expect

3. Click the button to set the DoubleTextBox Collapsed
   Expect: Get 1 error, IntTextBox2 has the red box, DoubleTextBox disappear
   Actual: Get zero error, IntTextBox2 has the red box, DoubleTextBox disappear

4. Click the button again to set the DoubleTextBox Visible
   Expect: Get 2 errors, both IntTextBox2 and DoubleTextBox have the red box
   Actual: Get 1 error, both IntTextBox2 and DoubleTextBox have the red box

Can you help me? thanks