Parent of child's receives pinch gesture only if started on certain points

Grabbed pinch gestures of a parent with multiple child's only work if the gesture starts at certain points. To be more exact it seems the gesture only works if at least one starting point is on the parent or both starting points are on a single child. The attached screenshot shows starting points that are good(green) and bad(red). The example app that generated the screenshot is attached as well. On pinch the example app logs the total scale factor with qDebug.

If the PinchGesture is grabbed at the QMainWindow the problem does not occur.

Edit: The same thing happens for PanGestures (2 Point, for example QScroller set with TouchGesture)