I explored a little more and the problem seems to be related to the image scale itself and smart object transformation. and the difference is that - export as : resize as smart objet THEN rasterize for output -> bad result - save for the web : rasterize the smartobjet THEN resize -> good result
if you resize the smartoject you get the same bad result hence it's not artboard related let's close this one and reopen another ticket with smart object resizing problem