Reference: Custom ItemsSource property for a UserControl

I have a custom UserControl to display a set of aggregate data for the window it’s contained in (a list of downloads). The basics of the UserControl are

<Grid>
  <StackPanel>
    <ScrollViewer>
      <ItemsControl ItemsSource="{Binding Downloads}">
        .
        .
        .
      </ItemsControl>
    </ScrollViewer>
    <local:DownloadStatsControl ItemsSource="{Binding Downloads}"/>
  </StackPanel>
</Grid>

The behind code for the UserControl contains

    public DownloadStatsControl()
    {
      InitializeComponent();

      DataContext = this;
    }

    [Bindable(true)]
    public IEnumerable ItemsSource
    {
      get => (IEnumerable)GetValue(ItemsSourceProperty);
      set
      {
        if (value == null)
          ClearValue(ItemsSourceProperty);
        else
          SetValue(ItemsSourceProperty, value);
      }
    }

    // Using a DependencyProperty as the backing store for ItemsSource.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty ItemsSourceProperty
            = DependencyProperty.Register("ItemsSource", typeof(IEnumerable), typeof(DownloadStatsControl),
                                          new FrameworkPropertyMetadata(null, new PropertyChangedCallback(OnItemsSourceChanged)));

    private static void OnItemsSourceChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
      var control = sender as DownloadStatsControl;

      control?.OnItemsSourceChanged((FixedSizeQueue<DownloadState>)e.OldValue, (FixedSizeQueue<DownloadState>)e.NewValue);
    }

My challenge is the set for ItemsSource isn’t called. The constructor is called.

Of note: The code above is different from the referenced SO. When I wasn’t getting the set called, I referred to the ItemControl source and pulled it back to basics (i.e., just use IEnumerable).

What am I missing here?