Blender .obj import artifact, how to fix?

when i import a multipart mesh example this pengolan tent from export the blender .obj


if i do not toggle “Normals” is more smooth wich i like more for cloth feeling of the tent sail
but there apears a black shadow rectangle on one of the side and no matter if i upload it with or withoud texture or change the roblox color and material on it withoud a texture to the mesh it still stays there
is there a way to avoid this ?

if i do normals its fixed but the the cloth looks just super sharp