Surface tool shading differs from BoxMesh with same material. Why?

Question

I have two cubes in my scene. One is just a BoxMesh, the other I generated with SurfaceTool. I added the vertices and indices, and generated normals. Both meshes have the same StandardMaterial3D, but for some reason, the mesh I generated with surface tool (the top one) does not shade the way I want:

Does anyone have any suggestions as to why it shades like that, and not like the cube below it?
The top cube is generated according to the cube tutorial I found here, updated for Godot 4: https://randommomentania.com/2018/11/godot-surface-tool-tutorial/
(see "Making A Cube" section.)

Answer 1

For anyone who sees this in the future:

surface_tool.set_smooth_group(-1)

Solved my problem. From the docs:

Note: generatenormals takes smooth groups into account. To generate smooth normals, set the smooth group to a value greater than or equal to 0 using setsmoothgroup or leave the smooth group at the default of 0. To generate flat normals, set the smooth group to -1 using setsmooth_group prior to adding vertices.