QGradient with ObjectBoundingMode produces an incorrect result

#include <QGuiApplication>
#include <QImage>
#include <QPainter>

int main(int argc, char *argv[])
{
    QGuiApplication a(argc, argv);

    QImage img(200, 200, QImage::Format_ARGB32);
    img.fill(Qt::transparent);

    QPainter p(&img);
    p.setRenderHint(QPainter::Antialiasing);
    p.setTransform(QTransform(1, 0, -0.57735026919, 1, 0, 0)); // skewX(-30)

    QRadialGradient rg(0.5, 0.5, 0.5, 0.5, 0.5);
    rg.setCoordinateMode(QGradient::ObjectBoundingMode);
    rg.setColorAt(0, Qt::white);
    rg.setColorAt(1, Qt::black);

    QBrush brush(rg);
    brush.setTransform(QTransform(0.86602540378, 0.5, -0.5, 0.86602540378, 0, 0)); // rotate(30)

    p.setPen(Qt::NoPen);
    p.setBrush(brush);

    p.drawRect(100, 40, 110, 110);

    p.end();

    img.save("test.png");

    return 0;
}