I copied what you had done, and found something strange:
When I right-clicked on the SpatialMaterial
inside the Mesh then selected "Make unique", the panel flashed and looked like it worked, but it did the same thing that happened to you. However, if I selected the mesh and said "Make unique", then did the same thing for the spatial material inside the Mesh, it allowed me to change the albedo of the second cube.
I believe this happens because when you duplicated the MeshInstance
, it made a shallow copy, reusing the mesh from the first. By making the mesh unique, then its SpatialMaterial
, it worked.