Steps to reproduce:

1. Download and open EnterTheRoom project
2. Make sure that Progressive GPU is selected as the lightmapping backend
3. Make sure that progressive updates/prioritize view is enabled
4. Bake the lighting
5. Now change any lighting-related value (light intensity, color, lightmapping setting, etc.)
6. Rebake and observe the artifacts

Notes:

- Only affects the GPU lightmapper; CPU lightmapper is unaffected
- Clearing the baked data fixes the issue
- Only reproduces when Progressive Updates are enabled
- Does not reproduce in: 2019.4.1f1 (73dc222d7c2f), 2020.1.0b10 (d80b47d1592d), 2020.1.0b12 (9e6726e6ce12), 2020.2.0a15 (86048ae4b4bd), 2020.2.0a16 (5a1f66dd0d56), 2020.2.0a19 (8037ac78137e)
- Reproduces in: 2019.4.2f1 (20b4642a3455), 2019.4.4f1 (1f1dac67805b), 2019.4.8f1 (60781d942082), 2020.1.0b13 (3682d55b6e87), 2020.1.0b14 (69bacbd35688), 2020.1.0b15 (40d9420e7de8), 2020.1.1f1 (2285c3239188)