I didn't have premultiplied clicked.  Is that how that function is supposed to work?

I find it an interesting question if there is something about the alpha channel that is too non-standard and/or advanced for PhotoShop to "see."  I was able to see the other g-buffers normally, but the color channels all contained this extra element.

Well, I can do everything I'd normally do with alpha channel with object index or another of the g-buffers instead.  Still annoying, though.