Ok, hopefully this is the final solution to the brightness differences:
Please revert all the changes in normals filtering, or any other advise I shared before. It shouldn't affect the issue.
Reload the texture you are using for your normals, but in the 3ds Max "Select Bitmap" dialog click on the "data image" button and only then open it. This will load the texture in "raw" mode and it should appear correctly, with no brightness variations. You may need to change the settings in your Corona Normal so that the shading is correct. For example, you may need to disable the "add gamma to input" option. I always test this by applying the normal to some simple object and placing a strong light next to it, to see if the shading is ok.
My understanding is that the colors of the normal map were not interpreted correctly and instead of "generally facing upwards" it would render as "generally facing a specific direction", which resulted in shading differences when it was rotated. So this is yet another fun exercise thanks to the 3ds Max color management.
PS. in the attached images Mapping Randomizer blending is set to 0, that's why tiles are visible.
