Jira: https://jira.unity3d.com/browse/GFXGI-489

Steps to reproduce:

1. Download repro project (link in the comments)
2. Open FAE_Demo scene
3. Navigate to the Lighting window
4. Select Progressive CPU as the lightmapping backend
5. Make sure that Progressive Updates is enabled
6. Bake the lighting
7. Upon bake completion, switch to Baked Lightmap scene view
8. Notice how some LODs are rendered black

Notes:

- Only affects the CPU lightmapper; GPU lightmapper is unaffected
- Does not reproduce upon clearing the baked data and rebaking
- Reproduces in: 2021.1.0a9, 2021.1.0a5, 2021.1.0a2
- Does not reproduce in: 2021.1.0a1, 2020.2.0b14