Elements of list that is bound in a ControlTemplate only rendered once

Question

I have a control that is wrapping an Xceed DataGridControl (part of the Extended WPF Toolkit Community Edition). The control provides a simple property (without a backing dependency property) that can hold a list of buttons (field instantiated by the constructor):

public List<Button> GroupButtons
{
    get { return groupButtons; }
    set { groupButtons = value; }
}

The items of the property are then added in the XAML of a view that is using the control:

<local:CustomControl ...>

    <local:CustomControl.GroupButtons>
        <Button>foo<Button>
    </local:CustomControl.GroupButtons>

    ...
</local:CustomControl ...>

I would like to render the buttons of this list inside the so-called "GroupHeaderControl" of the Xceed Datagrid, which is basically a grouping row like shown below:

To achieve this, I've overwritten the ControlTemplate of the GroupHeaderControl:

<ResourceDictionary ...>
    <Style TargetType="{x:Type controls:CustomControl}">
        <Style.Resources>
            <Style TargetType="{x:Type xcdg:GroupHeaderControl}">
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type xcdg:GroupHeaderControl}">
                            <Border ...>
                                <StackPanel Height="{TemplateBinding Height}" Orientation="Horizontal"> 

                                    <ContentPresenter  />                        
                                    <ItemsControl ItemsSource="{Binding GroupButtons,  RelativeSource={RelativeSource AncestorType={x:Type controls:CustomControl}}}" />

                                </StackPanel>
                            </Border>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </Style.Resources>
    </Style>
    ...
</ResourceDictionary>

Now here comes the problem: Instead of rendering the button(s) for each instance of the GroupHeaderControl, it is rendered only once. For illustration, imagine that in the image above only the button at the second group header ("Lyon") is visible while the other one ("Reims") is not.

The problem is apparently related to the fact that the items of the GroupButtons list are added via the XAML definition. If I hard code the items of the list, it works like a charm:

public List<Button> ButtonList
{
    get { return new List<Button>()
    {
        new Button() { Content = "foo" }
    }
}

I don't really get where this behavior is coming from. Does somebody have an idea?