Incorrect resutl of PenJoinStyle::RoundJoin

#include <QApplication>
#include <QPainter>

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

    QImage img(128, 128, QImage::Format_ARGB32_Premultiplied);
    img.fill(Qt::transparent);

    QPainterPath path;
    path.moveTo(100, 20);
    path.lineTo(100, 80);
    path.lineTo(20, 50);
    path.lineTo(100, 20); // back to start without ClosePath

    QPen pen(QColor("#39D"), 15);
    pen.setJoinStyle(Qt::RoundJoin);

    QPainter p;
    p.begin(&img);
    p.setRenderHint(QPainter::Antialiasing);
    p.setPen(pen);
    p.drawPath(path);
    p.end();

    img.save("test.png");

    return 0;
}