I think this is not a bug in clipping. Or at least not a bug in clipping only. QRect suffers from a backward compatibility "feature" and you might be experiencing it right now. Quoting the docs:
QPoint QRect::bottomRight () const
Returns the position of the rectangle's bottom-right corner.
Note that for historical reasons this function returns QPoint(left() + width() -1, top() + height() - 1).
The thing you experience might be somehow related to that (don't ask me how, I have no idea), although it's a long shot (but it looks like an off-by-one issue).

What happens if you first fill the pixmap with transparency? Is the image still black? Or is it transparent?