In short, this is an edge case of the current implementation. Everything is currently working as designed.
More specifically, SSAO and decals both need Depth+Normals and this causes a DepthNormals pass to run, but this replaces a Depth Copy pass that would produce correct results. In this particular case, because the DepthNormal pass is put in place of CopyDepth, and because the overlay camera's culling mask doesn't include the layer containing the quad, no depth information is ever made available to properly render the decal. Again, this is strictly a limitation of the current implementation, and everything is working as designed.