Let me start off by saying, I am not certain if this is a Blender problem, or a problem with Roblox rendering. They're having no luck finding the problem there, so I'm coming here to see if it's something Blender-related.
I have a model which loads up in reverse of how I need it to be. The left side is on the right. So, in Blender, I have mirrored it, and flipped the normals.
The grey brick is a light source in these two screenshots.
I've tried extruding, I've tried using -1 Z size instead of the mirror function, I've tried flipping and recalculating normals every way I can. But the lighting only looks right when I don't flip the Z axis at all, like so:
Is there a function in Blender that can fix this, or is it not a problem with the model, but the rendering program? (Blender's render shows no difference in lighting.)