QTransform results in distorted image for some images

consider the following code:

QImage image(...);
QPixmap pix = QPixmap::fromImage( image.transformed(QTransform().rotate(45) ,Qt::SmoothTransformation) );
pix.save(...);

This code results in a 45° rotated image for all images (attached), except for "test_800x600_8bit.png". Which results in a completely distorted image:

So it seems it fails for some images. But i only encountered it for some 8bit PNG files, which are bigger than a certain size, since it works for small 8bit PNG images.