My guess would be that it’s because that bottom edge isn’t marked sharp like the others. Any n-gon will look fine if it’s perfectly flat, but that applies to the normals as well as the actual geometry (refresher on that). Because the shading is smooth along that edge, it’s interpolating between the flat face and the angled face. Marking it sharp will separate the two so that the angles of each are not blended together when calculating the shading.
