understanding how normals works will tell you what you need to do, stwert said why, although it's not an artifact, it's correct for it to look like that, basically normals on all points of the face will face the same direction, but when you have a 'soft' edge the normals aren't all facing the same way, they point in other directions to make the edge look smooth. normals determine how the light bounces off the surface and affects how dark(?) it is at that point

but to resolve it without hardening the edges you'd have to insert more edges