1) What happened

Inspector preview incorrectly adds invalid color data to mesh even when material/shader doesnt require them. Console is filled with warning messages "Shader wants normals, but the mesh doesn't have them".

2) How can we reproduce it using the example you attached

1. Open "Test" scene.
2. Enter play mode.
3. Select "GameObject".
4. Double-click "Mesh" value within "MeshFilter" component.
5. Notice that inspector reports "4 verts, 2 tris uv,colors" and that console window is filled with warning messages.