Compositing a layer set to "Add" mode that has black pixels over a transparent area below does not create expected transparency. Black pixels on the "added layer" remain opaque.

I am rendering multichannel EXRs out of Vray for Maya.

I’m bringing footage in with Alpha ignored for comping.

Alpha is then extracted separately to be applied to layered comp. 

I need to composite an optical effects “Glare” render layer using Add over the top of the RGB back to beauty layer stack.

Clipping the layer stack works either by applying the Alpha as a Stencil Alpha, or as a direct matte on each layer, resulting in transparency as expected.

Adding the Glare above the Alpha layer, unmatted, should retain these transparent areas, adding only the non-black pixels.

It does not, the black areas of the Glare layer remain 100 percent opaque.

I’ve tried a variety of different solutions, rebuilt the comp, brought the Glare layer into the comp both precomped and not, and tried various different ways of using the Alpha below the Glare layer. Nothing works. The Add calculation is simply not working properly.