How to reproduce:
1. Open project "Case_1315335" and load Scene "SampleScene"
2. Select the Cube GameObject in the Hierarchy
5. In the Inspector, under the Mesh Renderer's Materials section, add additional Materials
6. Open the Probuilder Window (in the Menu Bar click Tools->ProBuilder->ProBuilder Window)
7. Select the Cube and ProBuilderize it
9. View the Cube's Mesh

Expectęd results: The Cube Mesh does not have any additional Submeshes
Actual results: The Cube Mesh has the same number of empty Submeshes as Materials in the Mesh Renderer

Reproducible with: ProBuilder 4.1.0 (2018.4.32f1, 2019.4.21f1, 2020.3.0f1), 5.0.0-pre.7 (2021.1.0b10, 2021.2.0a7)
Not reproducible with: 3.10.1 (2018.4.32f1, additional Materials get removed after ProBuilderizing it)

Notes:
- Many actions from the ProBuilder Window show the same behavior, not only ProBuilderizing
- The created Submeshes are made of 0 triangles and 0 indices