Presence of QWebEngineView make widgets misbehave

Short story

The presence of a QWebEngineView on screen seems to restore the graphic buffers of other widgets to a previous state.
Qt 5.5.1 does not have this issue.

Long story

I have a QMainWindow with 3 buttons and a stack widget. The stack widget as 3 widgets and the first one of them contains a QWebEngineView.

The 3 buttons are instances of MyButton which inherits QAsbtractbutton. They are checkable and are members of a QButtonGroup.
The clicked() signal of each button is connected to a slot which calls setCurrentIndex(int) on the stack widget.
I've managed to track paint events by drawing in the button a unique identifier.

void MyButton::paintEvent(QPaintEvent *event)
{
	static int n = 0;
	++n;

	qDebug() << this << isChecked() << isDown() << n;
	QPainter p(this);

	if(isDown() || isChecked())
	{
		p.setPen(Qt::NoPen);
		p.setBrush(QColor("#428bca"));
		//p.setClipRect(event->rect());
		p.drawRect(event->rect());

		p.setPen(Qt::white);
	}
	else
	{
		p.setPen(QColor("#428bca"));
	}
	p.drawText(6, 28, text() + " " + QString::number(n));
	p.end();
}

At runtime, when the application launches I can click ob the button 3 and the 3rd widget of the stack is displays. Then I press the left mouse button on the button 1 and I can see that the button 1 is repainted with a given value of n (let's say 30), the I release the mouse button, the clicked() signal is emitted and the stack displays the 1st widget (the one with a QWebEngineView. So far so good, but the buttons also get repainted with a lower version of n (let's say 25) even if in the debug output I can see calls to painEvent with higher n values.

The attached zip is a sample that exhibits the problem.