Vista - Bug: objet property not set (sample included)

Hello,
I switch to beta 2 and found a strange bug with my code customizing the
expander template. First here is a code sample to demonstrate the bug (i'll
explain it after) :

<StackPanel>
<StackPanel.Resources>
<SolidColorBrush x:Key="SolidBorderBrush" Color="#FFFF0000" />

<Style TargetType="{x:Type Expander}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Expander}">
<ToggleButton>
<ToggleButton.Template>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<StackPanel Orientation="Horizontal">
<Canvas Width="24" Height="24" Margin="2,2,2,2">
<Ellipse Canvas.Left="1" Canvas.Top="1" Width="22"
Height="22" StrokeThickness="1" Stroke="#FFFF0000"/>
<Path Canvas.Left="8" Canvas.Top="9"
StrokeThickness="1" Stroke="#FF000000" Data="F1 M 0,0L 4,8L 8,0L 4,1 Z"/>
</Canvas>
<Canvas Width="24" Height="24" Margin="2,2,2,2">
<Ellipse Canvas.Left="1" Canvas.Top="1" Width="22"
Height="22" StrokeThickness="1" Stroke="{StaticResource SolidBorderBrush}"/>
<Path Canvas.Left="8" Canvas.Top="9"
StrokeThickness="1" Stroke="#FF000000" Data="F1 M 0,0L 4,8L 8,0L 4,1 Z"/>
</Canvas>
</StackPanel>
</ControlTemplate>
</ToggleButton.Template>
</ToggleButton>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

<Style TargetType="{x:Type ToggleButton}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<StackPanel Orientation="Horizontal">
<Canvas Width="24" Height="24" Margin="2,2,2,2">
<Ellipse Canvas.Left="1" Canvas.Top="1" Width="22"
Height="22" StrokeThickness="1" Stroke="#FFFF0000"/>
<Path Canvas.Left="8" Canvas.Top="9" StrokeThickness="1"
Stroke="#FF000000" Data="F1 M 0,0L 4,8L 8,0L 4,1 Z"/>
</Canvas>
<Canvas Width="24" Height="24" Margin="2,2,2,2">
<Ellipse Canvas.Left="1" Canvas.Top="1" Width="22"
Height="22" StrokeThickness="1" Stroke="{StaticResource SolidBorderBrush}"/>
<Path Canvas.Left="8" Canvas.Top="9" StrokeThickness="1"
Stroke="#FF000000" Data="F1 M 0,0L 4,8L 8,0L 4,1 Z"/>
</Canvas>
</StackPanel>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</StackPanel.Resources>

<Expander x:Name="myExpander" />
<ToggleButton x:Name="myToggleButton" />
</StackPanel>

This sample should display 4 ellipse (2 for the expander and 2 for the
togglebutton) with an arrow inside each but if you try it you will find that
the second ellipse of the expander doesn't have an arrow.
The difference between the 2 shapes is simple : the first ellipse has a
"constant" color #FFFF0000 and the second uses a SolidColorBrush with the
same color.
The togglebutton is there to demonstrate that the but doesn't occure if the
template is applied directly on it but occures only if it's applied on a
togglebutton inside another template.

I looked with the debugger to find why the arrow after the ellipse with a
SolidColorBrush doesn't render and i found that its properties are not sets !

You can use those strings in the watch window to see this :

expander first path (OK) : ((Path) ((UIElement)
this.FindName("myExpander")).GetVisualChild(0).GetVisualChild(0).GetVisualChild(0).GetVisualChild(1)).Data

expander second path (KO) : ((Path) ((UIElement)
this.FindName("myExpander")).GetVisualChild(0).GetVisualChild(0).GetVisualChild(1).GetVisualChild(1)).Data

togglebutton first path (OK) : ((Path) ((UIElement)
this.FindName("myToggleButton")).GetVisualChild(0).GetVisualChild(0).GetVisualChild(1)).Data

togglebutton second path (OK) : ((Path) ((UIElement)
this.FindName("myToggleButton")).GetVisualChild(0).GetVisualChild(1).GetVisualChild(1)).Data

So can someone a microsoft can look into this ?

Thanks,
Guillaume