What's wrong with this grouped combobox binding?

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XML

.NET

combobox

DataGrid

Binding

I have a datagridtemplatecolumn with a grouped combobox on it. The items are displayed correctly, but the headers are not, actually a blank space appears.

I changed the Header field to match the same as the displayed data and the header still appears empty. Is there a different binding in the headertemplate than in the itemtemplate?

The Combobox is binded to a listcollection view with two fields: Bienes_ID and TipoBienes_ID with grouping on TipoBienes_ID.

The main datagrid is binded to a dataview with several fields, including Bienes_ID.

<DataGridTemplateColumn x:Name="ColumnaCombo">
                      <DataGridTemplateColumn.CellTemplate >
                          <DataTemplate >
                              <TextBlock Text="{Binding Path=Bienes_ID}"></TextBlock>
                          </DataTemplate>
                      </DataGridTemplateColumn.CellTemplate>
                      <DataGridTemplateColumn.CellEditingTemplate >
                          <DataTemplate >
                              <ComboBox x:Name="ComboBienesAgrupados"  SelectedValuePath="Bienes_ID" ItemsSource="{Binding Path=ListaBienes, RelativeSource={RelativeSource AncestorType=Window},UpdateSourceTrigger=PropertyChanged }" SelectedValue="{Binding Bienes_ID}"  >
                                  <ComboBox.GroupStyle >
                                      <GroupStyle>
                                          <GroupStyle.HeaderTemplate>
                                              <DataTemplate >
                                                  <TextBox   Text="{Binding  Path =Bienes_ID }" Background ="AliceBlue" ></TextBox>
                                              </DataTemplate>
                                          </GroupStyle.HeaderTemplate>
                                      </GroupStyle>
                                  </ComboBox.GroupStyle>
                                  <ComboBox.ItemTemplate >
                                      <DataTemplate >

                                          <TextBlock Text="{Binding Path=Bienes_ID}"></TextBlock>
                                      </DataTemplate>
                                  </ComboBox.ItemTemplate>
                              </ComboBox>
                          </DataTemplate>
                      </DataGridTemplateColumn.CellEditingTemplate>
                  </DataGridTemplateColumn>

What I have tried:

I managed to show in the itemtemplate the fields TipoBienes_ID, so only the headertemplate is not binding correctly.