Shadow incomprehension

Hi,

Here is my model inside the editor :



and once I put a light in front of it :



It looks like the light is coming from behind, but my light is exactly in front of this stair.

So this black triangle shadow has nothing to do here, I tried everything but I can not dismiss it, I don't get the point.

It is a point light, without shadow.

I let in my scene only my stair and my light, I tried to modify everything on the light and on the model inspector, I do not have lightmapping, tried different shaders, etc...

If anyone has a clue, would be more than appreciated, because I am loosing my mind here

 

UVMAP problem under 3DS MAX, my bad.

Do not waste your time