DataGrid ComboBox Binding Issue with Selected or New Item

I am having issue with retaining selected item in a DataGridTemplate column ComboBox. I have the DataTemplate editable combobox column as the first column in the datagrid and next to it, I have a text column. The DataGrid is populated with data read from SQL stored procedure. Everything works fine, except when I select an item in the combobox and move to the text field and start typing in it, the Combo selection blanks out. It blanks out both for a new item or existing item. Oddly, this happens only the first time. When I reselect the ComboBox Value or add the new item again and go back to the text field, it does not blank out. I am running out of ideas and tried many combinations, but no luck so far. Here is my code:

This is how I am populating the DataGrid:

using (SqlCommand cmd = new SqlCommand())
{
    cmd.CommandText = "GetProducts";
    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Connection = sqlConn;

    var reader = cmd.ExecuteReader();
    var dt = new DataTable();
    dt.Load(reader);
    dt.Columns["ProductName"].AllowDBNull = true;
    dtProductCfgTable = dt;
    ProductCfgGrid.ItemsSource = dtProductCfgTable.DefaultView;
}

This is the declaration for ProductNamesList:

public List<string> ProductNamesList { get; set; }

XAML:

<DataGridTemplateColumn Header="ProductName">
   <DataGridTemplateColumn.CellTemplate>
      <DataTemplate>
        <ComboBox ItemsSource="{Binding ProductNamesList, 
                                RelativeSource={RelativeSource AncestorType=Window}}"  
                                SelectedItem="{Binding ProductName 
                                IsSynchronizedWithCurrentItem="False"  
                                BorderThickness="1.2 1.2 0 0" BorderBrush="Black" 
                                Background="LightCyan" IsEditable="True" />
      </DataTemplate>
   </DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>  
<DataGridTextColumn Binding="{Binding ShippingAddress}" 
                    Width="100" 
                    Header="ShippingAddress" 
                    Visibility="Visible"/>