Hi All,
Thanks for the additional information. I am not sure if the ceiling shadow associated with the default interior light is the same problem as the one associated with point lights, but it certainly could be.
I spoke to one of our software engineers, though, and learned that although they aren't "real objects", Added Lights do have a bounding box of some kind, and the surfaces of that box are capable of casting shadows when they interact with the surface of an actual object - basically, it's a type of Z fighting. It looks like you can avoid that problem if the Added Light in question is about 40" or more away from any surfaces.
I hope that helps - and thank you again for the additional details.