Unexpected docking behavior after adding tool window to existing AutoHide-Group

We discovered some unexpected behavior when adding a new tool window to an existing AutoHide-Group.

Sample Code (new WPF project):

MainWindow.xaml:

<DockPanel>

        <Button DockPanel.Dock="Bottom" Content="Add" Click="ButtonBase_OnClick" />

        <docking:DockSite x:Name="DockSite" />

</DockPanel>

MainWindow.xaml.cs:

public partial class MainWindow
    {
        public MainWindow()
        {
            InitializeComponent();

            new ToolWindow(DockSite) {Title = "Tool"}.AutoHide(Dock.Left);
        }

        private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
        {
            new ToolWindow(DockSite) {Title = "Tool2"}.Dock(DockSite.ToolWindows[0], Direction.Content);
        }
    }

 When running this sample, clicking the Add-button and then the "Auto Hide"-Pin to dock the two tool windows works as we'd expect it.

-> Both tool windows are displayed within the same container.

But the following steps produce an unexpected situation:

1. Start the sample
2. dock the first tool window (by clicking the "Auto Hide"-Pin)
3. hide the window again (clicking "Auto Hide"-Pin again)
4. Add the second tool window with the Add-button
5. now dock the second tool window (by clicking the "Auto Hide"-Pin)

-> The two tool windows are displayed side-by-side, not within the same container.

Is this a bug or is something important missing?

Thanks in advance

Stefan