QSGVertexColorMaterial ignores alpha values when on top of a rectangle with a color

See https://qt-project.org/forums/viewthread/32614 for an example

I'm not the person who asked that question, but I have exactly the same problem. QSGVertexColorMaterial ignores alpha values st for vertices when the widget is on top of a rectangle with a color. If the color of the rectangle is unset, then non-255 alpha values successfully blend to the default background color (white).