Hi there watabou, great generators and viewers.
however I am having the same issue as jsunandmax - I took the Json from MFCG and imported it here, exported the obj and opened it in blender so I could scale it, I then took it to table top simulator.
once in TTS I noticed the normals were crazy - most of them appearing inside out, but some flipped outside.
I hopped back into blender and there was no fast or easy fix I could work out to sort the normals.
I think this is happening because the meshes are @open@ the buildings have a roof, and walls, but no "floor" or base. would it be possible for you to have an option to make the meshes "solid" as then even if the normals are from some reason inverted, blender can recalculate them to be "outside". at the moment since the mesh is "open" it cannot figure out inside from outside and just flips some one way or another.