Surface shader: Wrong lighting when moving vertices

If you are literally moving the verts in the mesh, then you need to recalculate the normals (Mesh.RecalculateNormals is a built-in, if you don't want to do it by hand.) The thing is, if you are only rotating the verts, then why not just rotate the model? Everything is set up to do the math for you when you use Transform.rotate.

The shader has no good way of knowing which way a face is "aimed," so we tell it with normals. For example, normals on the "top" verts are pre-made pointing upish. If you twist the verts around so that your top verts are now facing forwards, those normals still say the face counts as pointing up, so lights wrong (and the new top still counts as pointing backwards.)

But why don't all rotated models light wrong? When you spin a model overall, using the rotation, that rotation is sent to the shader. The auto-part of the surface shader knows how to fix the normals based on rotation. So, surface shaders work fine with rotated models, they expect it and even calculate the "fixed" normals for a non-rotated model. FYI, original normals are modelView and fixed ones are worldView (if you have to look these things up.)

If all you are doing is a rotate on verts, and you can't rotate the whole model, you'll need to also rotate the normals by the same angle (could also use recalNorms, but it does the same thing, slower, since it starts from scratch.)