Lead developer of an FR system here: this issue is dynamic range, darker faces, as well as very pale faces as well as any face that the brightest portions of the face are not far from the darker portions of the face, neglecting highlights is difficult for facial recognition simply because there is less distinguishable detail in faces with less visible feature variation. I see the same behavior in very pale Irish, those from Finland, soft featured Asians, darker than coffee Indians, and Africans. Anyone with very little illumination variation, which is necessary to distinguish their features.