That is by design. When using the compositing mode, you should place the beauty pass over the background without multiplying it with alpha (assume it has premultipled alpha). The alpha only modulates the background, but it should not modulate the beauty pass to get a correct result. If there was solid alpha below illumination from illuminators, that would make light from one source (scene illuminator) occlude light from other sources (background photo plate) - that is not how light works.