Reproduction steps:
1. Open attached "Pro Builder Mesh Revert Repro.zip" project and Scenes/SampleScene scene
2. Select Tools/ProBuilder/ProBuilder Window
3. In the Scene view, change the shape of a ProBuilderCube
4. In the Project window, select Materials/TestMaterial1.mat and drag it on ProBuilderCube
5. Observe the shape of a ProBuilderCube

Expected result: Shape does not change
Actual result: Shape reverts to its default state

Reproducible with: 5.0.0-preview.7 (2019.4.17f1, 2020.2.1f1, 2021.1.0b1)
Not reproducible with: 4.2.3 - 4.3.1 (2019.4.17f1, 2020.2.1f1, 2021.1.0b1)
Could not test with: 2018.4.29f1 (could not downgrade)

Notes:
1. Issue is not reproducible when Material is changed in the Inspector
2. Shape reverts even when dragged Material has not been applied (after dragging the Material on a GameObject but not letting go the Mouse button)