ItemsPanelTemplate in XAML ignores [ContentProperty] attribute

I have a custom Panel where I declared a custom property to hold the content (I don't want to use Children for the content):

[ContentProperty(Name = "PanelContent")]
public class CustomPanel : Panel
{
    public static readonly DependencyProperty PanelContentProperty =
       DependencyProperty.Register("PanelContent", 
       typeof(Collection<UIElement>), typeof(CustomPanel), 
       new PropertyMetadata(new Collection<UIElement>(), null));

    public Collection<UIElement> PanelContent
    {
        get
        {
            return (Collection<UIElement>)GetValue(PanelContentProperty);
        }
    }
}

This works perfectly when used like this:

<CustomPanel>
   <TextBlock>A</TextBlock>
   <TextBlock>B</TextBlock>
</CustomPanel>

But when I want to use the panel as an ItemsPanelTemplate inside ItemsControl, the ContentProperty attribute is ignored and adds everything to the Children collection, not the PanelContent collection:

<ItemsControl ItemTemplate="{StaticResource ReviewTemplate}" ItemsSource="{Binding Reviews}">
   <ItemsControl.ItemsPanel>
      <ItemsPanelTemplate>
         <CustomPanel></CustomPanel>
      </ItemsPanelTemplate>
   </ItemsControl.ItemsPanel>
</ItemsControl>

This is not how it should work. According to the documentation:

An ItemsPanelTemplate object element should contain exactly one FrameworkElement-derived class that serves as the root element for items. In most cases this is a Panel-derived class. The expanded template serves as the parent for the realized items and there generally is more than one item. Therefore the XAML content property of the intended r