This question is older but because its found in google on top I want to help to solve this problem.
This strange behavior happens If you don’t properly convert a 32bit Normal-File to 8Bit in Photoshop.
When changing from 32bit to 8bit mode choose the method “Equalize Histogram” - all other methods result in this strange light behavior you posted above.
If the resulted normal map is too “flat” so adjust the exposure afterwards (play with Exposure + Gamma Correction).