Binding vs MultiBinding => Various results

I have TextBlockinside a StackPanel. Since I use TextTrimming, I need to set the width TextBoxto match StackPanel.ActualWidth.

<StackPanel HorizontalAlignment="Stretch">
    <TextBlock HorizontalAlignment="Left">
        <TextBlock.Width>
            <MultiBinding Converter="{StaticResource WidthConverter}">
                <MultiBinding.Bindings>
                    <Binding RelativeSource="{RelativeSource Self}" />
                    <Binding RelativeSource="{x:Static RelativeSource.Self}" Path="TemplatedParent.Parent.ActualWidth"  />
              </MultiBinding.Bindings>
             </MultiBinding>                
        </TextBlock.Width>
    </TextBlock>

My converter:

Public Class WidthConverter
    Implements IMultiValueConverter

    Public Function Convert(ByVal values() As Object, ByVal targetType As System.Type, ByVal parameter As Object, ByVal culture As System.Globalization.CultureInfo) As Object Implements System.Windows.Data.IMultiValueConverter.Convert

        Const TextBoxMarginRight As Double = 5

        Dim ParentWidth As Double = CType(CType(values(0), FrameworkElement).Parent, FrameworkElement).ActualWidth

        Dim ParentRelativeControlPosition As Point = CType(values(0), FrameworkElement).TransformToAncestor(CType(CType(values(0), FrameworkElement).Parent, Media.Visual)).Transform(New Point(0, 0))

        Dim Width As Double = ParentWidth - TextBoxMarginRight - ParentRelativeControlPosition.X

        If Width > 5 Then
            Return Width
        Else
            Return 0
        End If

    End Function

Why is this working correctly and not the code below? (using IValueConverterwith the same code):

My converter can get StackPanelbut ActualWidthalways zero

<TextBlock.Width>

    <Binding RelativeSource="{x:Static RelativeSource.Self}" Path="TemplatedParent.Parent.ActualWidth"  Converter="{StaticResource WidthConverter}" />          
</TextBlock.Width>