1. Open a new project
2. Create a Unity simple cube
3. In the Inspector, increase the size of the cube's material list from the Mesh Renderer component
4. Probuilderize the cube and inspect it

Result:
The new Probuilderized cube will have as many submeshes than there was material on it before the operation and those submeshes will have 0 triangles.
Expected:
By probuilderizing an object, we should keep the original material list but no create empty submeshes for each material