How to reproduce:
1. [PC1] Open a new project
2. [PC1] In Hierarchy, create Cube Prefab and delete it from the Hierarchy
3. [PC1] [PC2] Sync up the changes on both computers
3. [PC1] In Project window, select the Cube prefab
4. [PC1] In Inspector, change the Layer to a newly created Layer
5. [PC1] Push changes via Collab
6. [PC2] Sync changes
7. [PC2] Observe the prefab's layer

Expected result: new layer is synced and set
Actual result: the layer field is empty, the new layer is not pushed

Reproducible with: 2018.4.19f1 (2.0.0-preview.15), 2019.3.6f1, 2020.1.0a21 (1.3.5, 1.3.6), 2020.2.0a3 (2.1.0-preview.3)
Could not test with: 2017.4 (errors in the console window)