QOpenGLPaintDevice and alpha other than 255

[Esperantisto]

If I render to an QOpenGLPaintDevice using the following code then everything looks fine:
@QOpenGLPaintDevice device(size);
QPainter painter(&device);

painter.setPen(QPen(QColor(255, 0, 0, 255), 5));
painter.setBrush(Qt::NoBrush);
painter.drawEllipse(QRect(size.width() / 2 - 100, size.height() / 2 - 100, 200, 200));@

Here's what it looks like:
!http://m2.i.pbase.com/o9/09/605909/1/154046342.nGGAoY9D.Screenshot_01.png!

If I change the alpha value of the color to anything other than 255 then here's what I get (128 used here):
!http://m4.i.pbase.com/o9/09/605909/1/154046344.QoJtHhK0.Screenshot_02.png!

Is this a limitation of QOpenGLPaintDevice or a bug?