这个问题来自于默认模板中用于标头的ContentPresenter。ContentPresenter的HorizontalAlignment设置为不为Stretch。要更改它，您可以尝试将Grid的HorizontalAlignment绑定到ContentPresenter的HorizontalAlignment。

<StackPanel Orientation="Vertical" x:Name="spTest" Margin="10">
    <Border BorderBrush="Black" BorderThickness="1">
        <Expander Margin="5,0,5,0">
            <Expander.Header>
                <Grid Margin="5,0,0,0" 

                    HorizontalAlignment="{Binding HorizontalAlignment, RelativeSource={RelativeSource AncestorType=ContentPresenter}, Mode=OneWayToSource}">

                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="Auto"/>
                        <ColumnDefinition Width="*"/>
                        <ColumnDefinition Width="40"/>
                    </Grid.ColumnDefinitions>
                    <TextBlock Text="TEXT1 1231521312"  Grid.Column="0" FontSize="18" Foreground="Black" VerticalAlignment="Center" HorizontalAlignment="Stretch"/>
                    <TextBlock Grid.Column="1" FontSize="18" FontStyle="Italic" Margin="10,0,10,0" Foreground="Black" VerticalAlignment="Center" Text="Text2"/>
                    <Button x:Name="btnRemove" Grid.Column="2" Width="40" Height="40" Background="#F8CECC" Content="X" FontWeight="Normal" FontSize="18" BorderBrush="#B95753"/>
                </Grid>
            </Expander.Header>
        </Expander>
    </Border>
</StackPanel>

另一种方法是设置/绑定Grid.Width，它可以处理嵌套的Expander。

<Grid Margin="5,0,0,0" Width="{Binding ActualWidth, RelativeSource={RelativeSource AncestorType=Expander}, Converter={StaticResource SubtractConverter}, ConverterParameter=5}">

public class SubtractConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return 
            (double)value 
            - 20/*Default template: Grid's column width */ 
            - 4/*Default template: margin of ContentPresenter*/ 
            - 2/*Default template:  2*BorderThikness */ 
            - double.Parse(parameter as string) /*Margin of Grid in Header*/;
    }

    public object ConvertBack(object value, Type targetTypes, object parameter, CultureInfo culture)
    {
        return value;
    }
}

另一种方法是覆盖Expander的默认模板，参见Expander Styles and Templates。