raster engine does not create deep copies when painting on QPixmap with QPainter

In the code below a pixmap is created. We then create a copy of the pixmap and save it as QImage. The pixmap is then modified and a second copy is created.
With the native graphicssystem both images are different as expected - with the raster graphicssystem both images have the same content. No deep copy is created.
If e.g. p->fill() is called a second time after the first copy has been made the deep copy is created as expected.

QPixmap *p = new QPixmap(300, 300);
p->fill(QColor(255, 255, 255, 0));
QPainter *myPainter = new QPainter(p);

QImage img1 = p->toImage();
img1.save("/tmp/img1.png");

myPainter->setBrush(Qt::black);
myPainter->drawEllipse(10, 10, 100, 100);

QImage img2 = p->toImage();
img2.save("/tmp/img2.png");

if (img1 == img2)