Rhino GUI do not show WPF Expander.Header content when it is not only text

When changing code from:

<Expander Header="Demand Points"/>


        <StackPanel Orientation="Horizontal" >
            <TextBlock  Text="Demand Points"/>

then Rhino do not display Exapnder header any more. It work with normal WPF application.

TextBlock: what style is associated?
Could the font size be zero?
Could the font color be the same as the background?
Could the size of the StackPanel zero? (use a Border with a color to check this)

Never trust the Visual Studio designer, it usually shows different results compared to runtime.

I used same code in WPF Application and that behaved correctly in runtime.

Even following code do not work. (That is only WPF / XAML code in project. There can not be any external referencing )

<UserControl x:Class="CFL.Panes.HDBPane"
             mc:Ignorable="d" Width="496" Height="485">