WPF Custom Control GroupBox - Empty Header

I'm trying to implement a Custom Control library project to be utilized in other projects. I'm 99% there and everything is mostly working, however, my custom Group Box control is being picky and not removing the header spacing to create a seamless box. Below is the XAML for the styling in Generic.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WPFCommonControls">
    <Style x:Key="GroupBoxStyleNoHeader" TargetType="{x:Type local:GroupBoxNoHeader}">
        <Setter Property="BorderBrush" Value="#D5DFE5"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:GroupBoxNoHeader}">
                    <Grid SnapsToDevicePixels="true">
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="6"/>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="*"/>
                            <ColumnDefinition Width="6"/>
                        </Grid.ColumnDefinitions>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="*"/>
                            <RowDefinition Height="6"/>
                        </Grid.RowDefinitions>
                        <Border Background="{TemplateBinding Background}" BorderBrush="Transparent" BorderThickness="{TemplateBinding BorderThickness}"  Grid.Column="0" Grid.ColumnSpan="4" Grid.RowSpan="3" Grid.Row="1"/>
                        <Border BorderBrush="White" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="4" Grid.ColumnSpan="4" Grid.RowSpan="3" Grid.Row="1">
                            <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
                                <Border BorderBrush="White" BorderThickness="{TemplateBinding BorderThickness}"/>
                            </Border>
                        </Border>
                        <Border x:Name="Header" Grid.Column="1" Padding="3,1,0,0" Grid.RowSpan="2" Grid.Row="0">
                            <ContentPresenter ContentSource="Header" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                        </Border>
                        <ContentPresenter Grid.Column="1" Grid.ColumnSpan="2" Margin="{TemplateBinding Padding}" Grid.Row="2" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

The style is mainly just the default GroupBox implementation that I pulled out and removed the OpacityMask in order to get rid of the header space. This method works 100% on a basic project where this is all local and contained in the a resource dictionary. However, now that I have an exterior project referencing my Custom Group Box project style, it's wigging out. The groupbox renders, however, the header space is not removed.

Here's my Custom Control's cs:

namespace WPFCommonControls
{
    /// <summary>
    /// Follow steps 1a or 1b and then 2 to use this custom control in a XAML file.
    ///
    /// Step 1a) Using this custom control in a XAML file that exists in the current project.
    /// Add this XmlNamespace attribute to the root element of the markup file where it is 
    /// to be used:
    ///
    ///     xmlns:MyNamespace="clr-namespace:WPFCommonControls"
    ///
    ///
    /// Step 1b) Using this custom control in a XAML file that exists in a different project.
    /// Add this XmlNamespace attribute to the root element of the markup file where it is 
    /// to be used:
    ///
    ///     xmlns:MyNamespace="clr-namespace:WPFCommonControls;assembly=WPFCommonControls"
    ///
    /// You will also need to add a project reference from the project where the XAML file lives
    /// to this project and Rebuild to avoid compilation errors:
    ///
    ///     Right click on the target project in the Solution Explorer and
    ///     "Add Reference"->"Projects"->[Select this project]
    ///
    ///
    /// Step 2)
    /// Go ahead and use your control in the XAML file.
    ///
    ///     <MyNamespace:GroupBoxNoHeader/>
    ///
    /// </summary>
    public class GroupBoxNoHeader : GroupBox
    {
        static GroupBoxNoHeader()
        {
            //DefaultStyleKeyProperty.OverrideMetadata(typeof(GroupBoxNoHeader), new FrameworkPropertyMetadata(typeof(GroupBoxNoHeader)));
        }
    }
}

I commented out the DefaultStyleKeyProperty stuff since my control would no longer show up with it enabled. I feel like this may be part of the issue? However, if I keep it in, then the whole group box doesn't show up.

MainWindow in separate project that references the Custom Control project

<Window x:Class="TestWpfResources.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:CommonControls="clr-namespace:WPFCommonControls;assembly=WPFCommonControls"
        xmlns:local="clr-namespace:TestWpfResources"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">

    <Grid>
        <CommonControls:GroupBoxNoHeader Margin="30,30,30,30">
        </CommonControls:GroupBoxNoHeader>
    </Grid>
</Window>