ContentPresenter loosing it data binding after triggers invoked more than twice - wpf

Question

I have created an extended button with 2 different border styles invoked by triggers in XAML. Both share the same contentpesenter but after changing the border style more than twice the content in the contentpresenter fails to display.
Below is a link to the entire project with a test bed application that demonstrates the issue, I think the issue is somewhere in the XAML below but I cannot see why it breaks:
Sample Button App
<Style.Resources>
<ContentPresenter x:Key="ButtonContent" Margin="5" HorizontalAlignment="Center"
Content="{Binding Content}"/>
</Style.Resources>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Grid Margin="{Binding KeyMargin}">
<Grid Visibility="{Binding RectangleVisibility}">
<Grid.OpacityMask>
<VisualBrush Visual="{Binding ElementName=rectBorder}" />
</Grid.OpacityMask>
<Border x:Name="rectBorder"
CornerRadius="{Binding BorderCorners}"
BorderBrush="{TemplateBinding BorderBrush}"
Background="{TemplateBinding Background}"
BorderThickness="{Binding BorderThickness}"/>
<Viewbox Stretch="Fill"
StretchDirection="Both">
<ContentControl Content="{StaticResource ButtonContent}"/>
</Viewbox>
</Grid>
<Grid Visibility="{Binding EllipseVisibility}">
<Ellipse Stroke="{TemplateBinding BorderBrush}"
StrokeThickness="{Binding BorderThickness}"
Fill="{TemplateBinding Background}">
</Ellipse>
<Viewbox Stretch="Fill"
StretchDirection="Both">
<ContentControl Content="{StaticResource ButtonContent}"/>
</Viewbox>
</Grid>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>

Answer 1

The problem is most likely that you cannot have the same element (the ContentPresenter in this case) in more than one place in the visual tree, and in which one of the two grids it ends up is undefined, i.e., an implementation archetype of WPF.
To get the element duplicated this might work:
<ContentControl Content="{TemplateBinding Content}"/>
or in your case
<ContentControl Content="{TemplateBinding Content}" Margin="5" HorizontalAlignment="Center"/>
instead of a static resource. The <ContentPresenter/> syntax is pretty much an optimized shortcut for that (or you could set x:Shared="False" on the resource, but having a ContentPresenter as a static resource is as far as I know not how it is intended to be used)
If the Button content is a UIElement itself though, it will be used directly itself in the visual tree, i.e., twice and this wont work either. A better solution would be to just have the content once in the control template and change the visual appearance around it, e.g., using a trigger to set the Grid's OpacityMask.
Another remark is that your control template is very tightly bound to where the Button is used, with direct bindings to the current data context, which reduces its reusability. Some easy fixes is to use TemplateBinding instead of Binding for BorderThickness respectively Margin (instead of KeyMargin), since those are existing properties of the Button.
For better reusability and cleaner code you should consider looking into creating a custom control deriving from Button with dependency properties for BorderCorners, the desired visual state (ellipse vs rectangle) etc. You might also want to use triggers to get the mouse-over effects of the button etc. Have fun control templating!