How to reproduce:
1. Create a new Project
2. Open ProBuilder Window
3. Create a new Shape or a Poly Shape
4. Select the Created Shape and drag the Material into the Inspector Window
5. Edit The Shape (Vertices, Edges or Faces)

Expected result: the edited Shape retains its Material
Actual result: the edited Shape resets to Default Material

Reproducible with: 2017.1.5f1, 2017.2.3p2, 2017.4.8f1, 2018.1.9f1, 2018.2.2f1, 2018.3.0a7

Note: the bug with 2017.1.4p2, 2017.2.3p2, 2017.4.8f1 versions could be reproduced with Probuilder import from the Asset Store.
The issue could not be reproduced when material is dragged on to the Object.

Workaround: drag the Material onto the Object itself in the Scene View