setStyleSheet on MainWindow break gradients in ToolButtons

#include <QApplication>
#include <QMainWindow>
#include <QToolButton>
#include <QWidgetAction>

class GradientWidget : public QWidget
{
public:
	GradientWidget(QWidget* parent = nullptr) : QWidget(parent), m_gradient(0, 0, 1, 0)
	{
		setAutoFillBackground(true);

		m_gradient.setCoordinateMode(QGradient::StretchToDeviceMode);
		m_gradient.setColorAt(0.0, Qt::black);
		m_gradient.setColorAt(1.0, Qt::white);
		applyGradient();
	}

	void mousePressEvent(QMouseEvent *e) override
	{
		m_gradient.setColorAt(0.5, Qt::red);
		applyGradient();
	}

	void applyGradient()
	{
		auto p = palette();
		p.setBrush(backgroundRole(), m_gradient);
		setPalette(p);
	}

	//reapplying gradient on showEvent fix it
	//void showEvent(QShowEvent *e) override
	//{
	//	applyGradient();
	//}

protected:
	QLinearGradient m_gradient;
};

int main(int argc, char * argv[])
{
	QApplication qapp(argc, argv);
	QMainWindow mainWindow;

	auto toolButton = new QToolButton();
	toolButton->setText("Test");

	QWidgetAction *widgetAction = new QWidgetAction(&mainWindow);
	auto w = new GradientWidget();
	w->setFixedSize(50, 20);

	widgetAction->setDefaultWidget(w);
	toolButton->addAction(widgetAction);

	mainWindow.setCentralWidget(toolButton);

	mainWindow.setStyleSheet("/* */"); //comment out this line helps too

	mainWindow.show();

	qapp.exec();
}