Accordion control showing item content only after second selection - wpf

Question

I have an AccordionControl (WPFToolkit) to which I add items dynamically:
<my:Accordion Grid.Column="1"
ItemsSource="{Binding Path=Tests}"
SelectionMode="ZeroOrOne"
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch">
<my:Accordion.ItemTemplate>
<DataTemplate>
<StackPanel>
<TextBlock Text="{Binding Header}" />
</StackPanel>
</DataTemplate>
</my:Accordion.ItemTemplate>
<my:Accordion.ContentTemplate>
<DataTemplate>
<ContentPresenter Content="{Binding Content}"/>
</DataTemplate>
</my:Accordion.ContentTemplate>
</my:Accordion>
To actually see the Content, I have to select (open) an AccordionItem, close it and open it again. What could be the reason for this behaviour?
EDIT
I have found a way around this using styles instead, still I would be interested why the above does not work. Here the style solution:
<Style x:Key="itemStyle" TargetType="my:AccordionItem">
<Setter Property="HeaderTemplate">
<Setter.Value>
<DataTemplate>
<TextBlock Text="{Binding Header}"/>
</DataTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="accordionStyle" TargetType="my:Accordion">
<Setter Property="ItemContainerStyle" Value="{StaticResource itemStyle}" />
<Setter Property="ContentTemplate">
<Setter.Value>
<DataTemplate>
<ItemsControl ItemsSource="{Binding MenuItems}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBox Text="{Binding Path=Text, Mode=OneWay}"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</DataTemplate>
</Setter.Value>
</Setter>
</Style>
<my:Accordion Grid.Column="1" Height="Auto"
Name="accordion1"
ExpandDirection="Left"
SelectionMode="One"
ItemsSource="{Binding Tests}"
Style="{StaticResource accordionStyle}">
</my:Accordion>
EDIT
I have now found the problem: I cannot "Stretch the AccordionControl.
<my:Accordion Grid.Column="1" Height="Auto"
VerticalAlignment="Stretch"
HorizontalAlignment="Stretch"
Name="accordion1"
ExpandDirection="Left"
SelectionMode="One"
ItemsSource="{Binding Tests}"
Style="{StaticResource accordionStyle}">
</my:Accordion>
As soon as I do this, it is not working anymore. Does someone know a way around this?