WPF custom slider PART_SelectionRange lags behind the thumb

I've created custom slider and thumb templates for my app. It's working as expected except for the PART_SelectionRange Rectangle, which begins slightly behind the thumb and by the time the maximum value is reached is noticeably far behind it.

I have no clue how this could be fixed since it looks like the problem is in the behaviour of whatever class is controlling the size of the PART_SelectionRange Rectangle.

(Sorry, I don't know how to inline it).

<ControlTemplate x:Key="SliderThumbHorizontalDefault" TargetType="{x:Type Thumb}">
    <Grid>
        <!-- SliderThumb -->
        <Path x:Name="grip" Data="M-0.4,6.4 C1.2,1 9,1 10.5,6.5 12,11.787571 5.0267407,18.175417 5.0267407,18.175417 5.0267407,18.175417 -2,11.8 -0.4,6.4 z"
            Fill="White" Stretch="Fill" SnapsToDevicePixels="True"
            Stroke="Black" StrokeThickness="1" UseLayoutRounding="True" />
        <ContentPresenter Margin="0 4 0 0"  Content="{Binding Value, RelativeSource={RelativeSource AncestorType={x:Type Slider}}}" TextBlock.Foreground="Black" TextBlock.FontSize="20" TextBlock.TextAlignment="Center" />
    </Grid>
</ControlTemplate>

<ControlTemplate x:Key="SliderHorizontal" TargetType="{x:Type Slider}">
    <ControlTemplate.Resources>
        <vc:TickConverter x:Key="TickConverter" />
    </ControlTemplate.Resources>
    <Grid Height="{StaticResource SliderHeight}">
        <Border x:Name="TrackBackground" BorderBrush="{StaticResource SliderThumb.Track.Border}" BorderThickness="0"
                Background="White"
                CornerRadius="{Binding ActualHeight, Converter={StaticResource HalvingConverter}, ElementName=TrackBackground}">
            <!-- This rectangle is what lags behind! -->
            <Rectangle x:Name="PART_SelectionRange" Fill="SlateBlue"
                        HorizontalAlignment="Left" RadiusY="3" RadiusX="3" />
        </Border>
        <Track x:Name="PART_Track" >
            <Track.Thumb>
                <Thumb x:Name="Thumb" Focusable="False" Width="35" Height="47" OverridesDefaultStyle="True"
                            Template="{StaticResource SliderThumbHorizontalDefault}" Margin="-14.65,-55,-16,12"/>
            </Track.Thumb>
        </Track>
    </Grid>
    <ControlTemplate.Triggers>
        <Trigger Property="IsSelectionRangeEnabled" Value="true">
            <Setter Property="Visibility" TargetName="PART_SelectionRange" Value="Visible" />
        </Trigger>
        <Trigger Property="IsKeyboardFocused" Value="true">
            <Setter Property="Foreground" TargetName="Thumb" Value="Blue" />
        </Trigger>
    </ControlTemplate.Triggers>
</ControlTemplate>

<Style TargetType="{x:Type Slider}">
    <Setter Property="MinHeight" Value="{StaticResource SliderHeight}" />
    <Setter Property="MaxHeight" Value="{StaticResource SliderHeight}" />
    <Setter Property="Stylus.IsPressAndHoldEnabled" Value="false" />
    <Setter Property="IsSelectionRangeEnabled" Value="True" />
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="BorderBrush" Value="Transparent" />
    <Setter Property="Foreground" Value="{StaticResource SliderThumb.Static.Foreground}" />
    <Setter Property="SelectionStart" Value="{Binding Minimum, RelativeSource={RelativeSource Self}}" />
    <Setter Property="SelectionEnd" Value="{Binding Value, RelativeSource={RelativeSource Self}}" />
    <Setter Property="Template" Value="{StaticResource SliderHorizontal}" />
</Style>

At this point I don't know what I should be trying to modify. I believe I've tried changing every property that made sense from xaml, so I think the problem is in the Slider class, but can't fathom where.