How to reproduce:
1. Open the attached '1289851.zip' project
2. Open the 'Test1' Scene
3. In the Hierarchy window select the 'Cube' object
4. Observe the 'Additional Vertex Streams' field in the Inspector window
5. Open the 'Test2' Scene
6. Open Window > Package Manager
7. Update the Polybrush package to the 1.0.2 version
8. Open the 'Test1' Scene
9. Select the 'Cube' object and observe the 'Additional Vertex Streams' field

Expected result: The Additional Vertex Streams value is not cleared
Actual result: The Additional Vertex Streams value is cleared

Reproducible with: 2019.4.14f1, 2020.1.14f1, 2020.2.0b11, 2021.1.0a5 (when upgrading the Polybrush package from 1.0.0, 1.0.1 to 1.0.2)
Not reproducible with: 2019.4.14f1, 2020.1.14f1, 2020.2.0b11, 2021.1.0a5 (when upgrading the Polybrush package from 1.0.0 to 1.0.1)
Cannot test with 2018.4.29f1 (Multi-object editing is not supported)

Notes:
- Sometimes after updating the package and opening the 'Test1' Scene the Polybrush Mesh (Script) component's 'Apply as' field changes to 'Overwrite Mesh' (1/3 times)