Disabled color of NumBox header

As NumBox seems to have a bug of not dimming the label when the control is disabled (other controls do, of course), I try to style it to force this behavior:

  <Style TargetType="ux:NumberBox">
    <Setter Property="HeaderTemplate">
      <Setter.Value>
        <DataTemplate>
          <TextBlock x:Name="HeaderText" Text="{Binding}" Foreground="{ThemeResource TextControlHeaderForeground}">
            <VisualStateManager.VisualStateGroups>
              <VisualStateGroup x:Name="VisualStateGroup">
                <VisualState x:Name="Disabled">
                  <VisualState.Setters>
                    <Setter Target="HeaderText.Foreground" Value="{ThemeResource SystemControlDisabledBaseMediumLowBrush}" />
                  </VisualState.Setters>
                </VisualState>
              </VisualStateGroup>
            </VisualStateManager.VisualStateGroups>
          </TextBlock>
        </DataTemplate>
      </Setter.Value>
    </Setter>
  </Style>

While the original Foreground is OK, it doesn't pick up the change in the visual state and the label stays the same color. What am I doing wrong?