QOpenGLWidget problem when superimposing a semi transparent QWidget

"Label Under" is QLabel with background color set to (128,128,128,255)
The Widget with the Teapot is a QOpenGLWidget with background color set to (128./255,128./255,128./255,1.)

"Label Over" is QLabel with background color set to (255,0,0,128). It displays over both the QLabel "Label Under" and the QOpenGLWidget

As one can see, final rendering of the "QOpenGLWidget" area differs from the QLabel "Label Under" area wheras we expect the same 'final' color.