Any chance of more info, such as the geometry the light material is applied to (a plane? Does it have anything unusual applied, e.g. to make it always face the camera? What is the mapping of the opacity map, just UVW, again, nothing more unusual that would involve the camera?) A look at renders might not hurt. Does it happen with a regular Corona Mtl, either with self illumination or just with diffuse, to see if the opacity map in general has this problem, or if it's specific to the light material.