In C++ `FindName` wont reload an object that has been `UnloadObject`ed

[tom-huntington]

Describe the bug

This doesn't happen in C#

This also happens if we replace the call to UnloadObject with myStackPanel.Children().ReplaceAll({});. This is similar to https://github.com/microsoft/microsoft-ui-xaml/issues/10176

Steps to reproduce the bug

<Window ...>
    <StackPanel x:Name="myStackPanel" Orientation="Vertical"  VerticalAlignment="Center">
            <Button x:Load="True" x:Name="myButton" Click="myButton_Click">Click Me</Button>
    </StackPanel>
</Window>

void MainWindow::myButton_Click(IInspectable const&, RoutedEventArgs const&)
{
    UnloadObject(myButton());
    winrt::check_bool(!myButton());
    auto r = myStackPanel().FindName(L"myButton");
    winrt::check_bool(!r); // wrong
    winrt::check_bool(!myButton()); // wrong
}

private void myButton_Click(object sender, RoutedEventArgs e)
{
    UnloadObject(myButton);
    Debug.Assert(myButton == null);
    var r = myStackPanel.FindName("myButton");
    Debug.Assert(r != null); // good
    Debug.Assert(myButton != null); // good
}

NuGet package version

WinUI 3 - Windows App SDK 1.6.3: 1.6.241114003

Windows version

Windows 11 (21H2): Build 22000

Additional context

No response

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[tom-huntington]

https://learn.microsoft.com/en-us/windows/uwp/xaml-platform/x-load-attribute

when the element is not loaded, its memory is released and internally a small placeholder is used to mark its place in the visual tree.

So I'm guessing this placeholder is being removed. When I iterate over Panel.Children the placeholders are not there. But calling UIElementCollection.ReplaceAll removes them, as per my other issue