Problem with transparent background at TextVisual3D

Hello,

is it correctly that the TextVisual3D only display the transparent backgorund from last insert TextVisual3D?

        <helix:HelixViewport3D ZoomExtentsWhenLoaded="True" ShowCoordinateSystem="True"  CameraMode="Inspect">
            <helix:SunLight />
            <helix:TextVisual3D Position="1,-1,1" Text="Hello"  Foreground="Aqua" Background="Transparent" UpDirection="0,0,1"  Height="1"/>
            <helix:GridLinesVisual3D Width="10" Length="10" MinorDistance="1" MajorDistance="1" Thickness="0.01"/>
            <helix:SphereVisual3D Center="0,0,0" Radius="0.5" />
            <helix:EllipsoidVisual3D Center="3,3,3" RadiusX="2" RadiusY="3" RadiusZ="4" />
        </helix:HelixViewport3D>

        <helix:HelixViewport3D ZoomExtentsWhenLoaded="True" ShowCoordinateSystem="True"  CameraMode="Inspect">
            <helix:SunLight />
            <helix:GridLinesVisual3D Width="10" Length="10" MinorDistance="1" MajorDistance="1" Thickness="0.01"/>
            <helix:SphereVisual3D Center="0,0,0" Radius="0.5" />
            <helix:EllipsoidVisual3D Center="3,3,3" RadiusX="2" RadiusY="3" RadiusZ="4" />
            <helix:TextVisual3D Position="1,-1,1" Text="Hello"  Foreground="Aqua" Background="Transparent" UpDirection="0,0,1"  Height="1"/>
        </helix:HelixViewport3D>

If flip in view from backside is transparency not work on other TextVisual3D.

        <helix:HelixViewport3D ZoomExtentsWhenLoaded="True" ShowCoordinateSystem="True"  CameraMode="Inspect">
            <helix:SunLight />
            <helix:GridLinesVisual3D Width="10" Length="10" MinorDistance="1" MajorDistance="1" Thickness="0.01"/>
            <helix:SphereVisual3D Center="0,0,0" Radius="0.5" />
            <helix:EllipsoidVisual3D Center="3,3,3" RadiusX="2" RadiusY="3" RadiusZ="4" />
            <helix:TextVisual3D Position="1,1,1" Text="Hello"  Foreground="Aqua" Background="Transparent" UpDirection="0,0,1"  Height="1"/>
            <helix:TextVisual3D Position="0.5,1.5,1" Text="Hello"  Foreground="Aqua" Background="Transparent" UpDirection="0,0,1"  Height="1"/>
        </helix:HelixViewport3D>

How can i fix it?

regards Mario