ESlateBrushDrawType::Box not work in SProgressBar

// Progress Bar
+ SOverlay::Slot().HAlign(HAlign_Fill).VAlign(VAlign_Bottom).Padding(FMargin(0, 0, 0, 20))
[
SNew(SVerticalBox)

	// CircularThrobber
	+ SVerticalBox::Slot().HAlign(HAlign_Center).Padding(FMargin(0, 0, 0, 15)).AutoHeight()
	[
		SNew(SCircularThrobber)
		.Radius(20.f)
		.Visibility(this, &SLoadingScreenWidget::GetLoadIndicatorVisibility)
	]

	// Progress Bar
	+ SVerticalBox::Slot().HAlign(HAlign_Center).AutoHeight()
	[
		SNew(SBox).WidthOverride(m_vProgressSize.X).HeightOverride(m_vProgressSize.Y)
		[
			SAssignNew(m_pProgressBar, SProgressBar)
			.BarFillType(EProgressBarFillType::LeftToRight)
			.Percent(m_fProgressGauge)
			.BorderPadding(FVector2D(2.f, 2.f))
			.BackgroundImage(m_pProgressBgBrush.Get())
			.FillImage(m_pProgressFillBrush.Get())
		]
	]
]

m_pProgressBgBrush ESlateBrushDrawType is Box.
and m_pProgressFillBrush is Box too.

but they work like ESlateBrushDrawType::Image

like this

Please help me.

Why ESlateBrushDrawType::Box not work in SProgressBar?