Qt fails to flush to Mac native sub window when tlw is RHI

#include <QtWidgets/QApplication>
#include <QtWidgets/QWidget>
#include <QtOpenGLWidgets/QOpenGLWidget>
#include <QtCore/QTimer>
#include <QtCore/QPointer>
#include <QtGui/QBackingStore>
#include <QtGui/QPainter>

// This can be any custom native WA_PaintOnScreen widget. e.g. a custom Metal canvas
class SomePaintOnScreen : public QWidget
{
public:
	SomePaintOnScreen(QWidget* parent)
		: QWidget(parent)
	{
		setAttribute(Qt::WA_OpaquePaintEvent);
		setAttribute(Qt::WA_PaintOnScreen);
		winId();
	}

	QPaintEngine* paintEngine() const override
	{
		return nullptr;
	}
	
	void paintEvent(QPaintEvent* pe) override
	{
		// This is only some convenient code to fill NSView with green pixels.
		// Can be replaced by any code to manipulate NSView.
		// e.g. set a custom CAMetalLayer and render some 3d contents to it.
		QBackingStore store(windowHandle());
		store.resize(size());
		store.beginPaint(rect());

		QPainter p(store.paintDevice());
		p.setCompositionMode(QPainter::CompositionMode_Source);
		p.fillRect(rect(), Qt::green);
		p.end();

		store.endPaint();
		store.flush(rect(), windowHandle(), QPoint(0, 0));
	}
};

int main(int argc, char** argv)
{
	QApplication app(argc, argv);

	// tlw: red.
	QWidget tlw;
	tlw.resize(640, 480);
	tlw.setStyleSheet("QWidget { background-color: red; }");

	// gl: black.
	QOpenGLWidget gl(&tlw);
	gl.setGeometry(0, 0, 640, 240);
	
	// tlw's window's surface type will become OpenGL because there is a
	// QOpenGLWidget child. If there is a quick child, it will become
	// Metal by default.
	// In both case, tlw's backing store is QRhiBackingStore.
	
	// paint-on-surface: green.
	SomePaintOnScreen canvas(&tlw);
	canvas.setGeometry(0, 240, 640, 240);

	// native child of paint-on-surface: blue.
	QWidget raster(&canvas);
	raster.setGeometry(0, 0, 640, 240);
	raster.setStyleSheet("QWidget { background-color: blue; }");
	
	// We expect:
	//   [ black ]
	//   [ blue ]

	// Got:
	//   [ black ]
	//   [ green ]
	
	// QOpenGLWidget cause the tlw's backing store becomes QRhiBackingStore.
	// QRhiBackingStore is not able to flush into a native raster child.
	//

	tlw.show();
	return app.exec();
}