Well, after looking several times, trying and trying again I think this could be an issue.
I have loaded an object than exported again as I changed it and I'm not able to refresh it, just like if it always uses a cached version without updating it.
If I create a new mesh object and load the last exported version, I indeed get the good version, so I have tow meshes pointing to the same obj file but the result is different ...
Hope this is clear
Edit : Forget it, my bad, it was due to a bad UV unwrapping