Why seperating a sphere gives different shades?

Question

I started blender couples months ago, and I wanted a character with a torso similar to Mario (round). I imported my character on Roblox studio but then I saw that the lower part is a little bit darker than the upper part of the torso. I recreate the torso thinking that I accidentally did something wrong, but same issue. I thought that the problem came from roblox and I did a little bit of research but I found that the problem is about 'recalculating outside normals', but unfortunately, nothing change.

But then I saw that separating a cube is different than separate a sphere. The cube make it less obvious that its two part, but each times I separate a sphere but smaller part becomes more darker.

Also, I have no lighting, when I paint my mesh with vertex paint the same issue occurs (for example, I paint red and the lower part is a darker red), I also tried with material but same issue.

Answer 1

The shading is determined by the split normals, you can display them in the Mesh Edit Mode Overlays panel (in the latest Blender versions). There's 1 split normal per vertex and its direction is determined by the face(s) it is part of, if you rip an edge it will create 2 vertices instead of one, and their split normals will diverge as the faces they are part of are not connected anymore:

That said you can still make the split normals converge again, to do that put the 2 spheres at the same location, give to your ripped sphere a Data Transfer modifier with the original sphere as Source, enable Face Corner Data, click on Custom Normals, click on Generate Data Layers button, apply the modifier, the split normals of the new sphere will align with the original one which will recreate the same shading: