How to reproduce:
1. Open attached "GPULightmapperRepro.zip" and Scenes/SampleScene scene
2. In the Hierarchy, select Ford_environment Game Object
3. Change Scene view to Texel Validity
4. Select Window/Rendering/Lighting Settings and Generate Lighting
5. Change Environment Samples to 64(or make other changes) and Generate Lighting
6. Observe the Scene view

Expected result: The Texel validity view is green
Actual result: The Texel validity view is half red

Reproducible with: 7.3.1-7.5.1(2019.4.14f1), 8.2.0(2020.1.13f1)
Not reproducible with: 8.2.0(2020.1.14f1, 2020.1.15f1), 10.1.0(2020.2.0b11, 2021.1.0a6, 2021.1.0a7)
Could not test with: 2018.4.29f1(URP not supported)

Notes:
1. Not reproducible when Generating Lighting with progressive CPU
2. Lightmap is baked properly if baked data is cleared before Generating Lighting