QOpenGLWidget's format(smaples) would be reset As QGraphicsView’s viewport

[SuperAo]

I try to enable MSAA in QOpenGLWidget, So I set the format samples as 8. Everything is fine in QGraphicsView's constructor. But MSAA does not work, then I find in drawcall function, format samplels == 0 ! Here are codes;
QGraphicsView's constructor code :

        auto gl_widget = new SGOpenGLWidget(this);
	QSurfaceFormat fmt = gl_widget->format();

	setViewport(gl_widget);
	auto view_widget = this->viewport();
	QOpenGLWidget* gg_widget = dynamic_cast<QOpenGLWidget*>(view_widget);
	if (gg_widget)
	{
		auto fmt = gg_widget->format();  
		auto num = fmt.samples(); // num is 8,bacause i set it as deafult format
	}

but at QGraphicsView::drawBackground(QPainter* painter, const QRectF& rect) , code is here:

auto view_widget = this->viewport();
		QOpenGLWidget* gl_widget = dynamic_cast<QOpenGLWidget*>(view_widget);
		if (gl_widget)
		{
			auto fmt = gl_widget->format();
			
			auto num = fmt.samples(); // samples num becomes 0! 
			fmt.setSamples(4);
			gl_widget->setFormat(fmt);// not work
			auto size = fmt.depthBufferSize();
			auto gl_ctx = gl_widget->context();
			auto ctx_fmt = gl_ctx->format();
			auto ctx_num = ctx_fmt.samples();
		}

So SMAA does not work at all. Is it a bug in Qt?