See attached screenshots. "opengl1.png" displays the correct behavior, and "opengl2.png" shows the bug. Note the blocks of non-transparent color around the glyphs. Note also that the rendering of the semi-transparent blue rectangle is darker in the opengl2 version, even though the only difference is the paint engine.

I arrived at this bug by creating a subclass of QFrame and overriding paintEvent to draw a translucent image instead of the normal frame. I then subclassed the resulting widget and added a QLabel over top of it.