Not sure if this will help or hinder you, but i have noticed similar issues when lighting is with spot lights.
Here are 2 screen shots showing what i did, the "lights" are just 2 cylinders 3" dia.
1st is with the light source 3 7/8" 10%
2nd is 1 7/8
Hope that helps, in some way, you identify the cause and find a solution.
Mike