Steps to reproduce:
1. Open the attached "case_1300585.zip" project and Game scene
2. Select Window/Rendering/Lighting
3. Set Progressive CPU as Lightmapper and Generate Lighting
4. Observe the Game view
Expected result: Lighting is rendered properly
Actual result: Artifacts are visible
Reproducible with: 2020.2.2f1, 2021.1.0b2, 2021.2.0a2
Could not test with: 2018.4.29f1, 2019.4.17f1(could not downgrade)