In case anyone finds this thread while looking for a solution to a “shading seam” that appears along an edge loop when you modularize/separate a moderate to high poly mesh up into several body parts e.g. separate a body into a head and a torso, as I have just done, the solution to the shading seam is to join the parts back together again, merge vertices by distance to remove those vertices with the different normals, split the edge loop where the seam is e.g. around the neck when splitting a body mesh into a torso and a head mesh, then separate the body parts up again into several objects. Explanation: The splitting of the edge loop where the seam is , creates 2 identical edge loops with identical normals. When subsequently separating the mesh into several objects (body parts) those normals are retained (stay the same) and thus the shading along that edge loop is retained.