Здравствуйте, igor-booch, Вы писали:
...
IB>RelativeSource Self я полностью убрал из кода. Дело не в этом.

Для пояснения пример:

UserControl без CodeBehind:

<UserControl x:Class="WpfApplication1CS.Window18UC1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:WpfApplication1CS"
             mc:Ignorable="d"
             d:DesignHeight="300" d:DesignWidth="300">
  <StackPanel>
    <Button ContentTemplate="{Binding ItemTemplateVM}"/>
  </StackPanel>
</UserControl>

XAML-Window:

<Window x:Class="WpfApplication1CS.Window18"
        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:local="clr-namespace:WpfApplication1CS"
        mc:Ignorable="d"
        DataContext="{Binding RelativeSource={RelativeSource Self}}"
        Name="window"
        Title="Window18" Height="300" Width="300">
  <Window.Resources>
    <DataTemplate x:Key="dt">
      <Label Content="{Binding Data.Name1, RelativeSource={RelativeSource AncestorType={x:Type Window}}}" />
    </DataTemplate>
  </Window.Resources>
  <StackPanel>
    <local:Window18UC1/>
    <Button ContentTemplate="{StaticResource dt}" Name="btn"/>
  </StackPanel>
</Window>

CodeBehind-Window:

using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;

namespace WpfApplication1CS
{
  /// <summary>
  /// Interaction logic for Window18.xaml
  /// </summary>
  public partial class Window18 : Window
  {
    public Window18()
    {
      InitializeComponent();
      this._data = new Window18Data() { Name1 = "Текст на кнопке" };
      this.ItemTemplateVM = btn.ContentTemplate;
    }

    private DataTemplate _itemTemplateVM;
    public DataTemplate ItemTemplateVM
    {
      get { return _itemTemplateVM; }
      set
      {
        _itemTemplateVM = value;
        OnPropertyChanged();
      }
    }
    private Window18Data _data;
    public Window18Data Data
    {
      get { return _data; }
      set
      {
        _data = value;
        OnPropertyChanged();
      }
    }
    #region OnPropertyChanged
    public event PropertyChangedEventHandler PropertyChanged;
    private void OnPropertyChanged([CallerMemberName] string propname = "") =>
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propname));
    #endregion
  }
  public class Window18Data
  {
    public string Name1 { get; set; }
  }
}