How to reproduce:
1. Open the user's attached "Lightmapping1.zip" project
2. Open the "Lighting" Scene
3. Make sure Scene view Draw Mode is set to Shaded
4. Press "Generate Lighting" in the Lighting window (Window -> Rendering -> Lighting)
5. Set the Scene view Draw Mode to Baked Lightmap
6. Press "Generate Lighting" in the Lighting window
7. Observe that the "Lightmap-0_comp_light" Asset
8. Re-open Unity Editor
9. Repeat 6-7 steps

Expected result: The "Lightmap-0_comp_light" Asset is blue (step 7) and the "Lightmap-0_comp_light" Asset stays blue after generating the lighting after re-opening the Unity Editor (step 9)
Actual result: The "Lightmap-0_comp_light" Asset is blue (step 7) and the "Lightmap-0_comp_light" Asset becomes dark yellow after generating the lighting after re-opening the Unity Editor (step 9)

Reproducible with: 2019.4.14f1, 2020.1.12f1, 2021.1.0a2
Could not test with: 2018.4.28f1 (Assets get corrupted when downgrading), 2021.1.0a3 ("IndexOutOfRangeException: Invalid kernelIndex (0) passed, must be non-negative less than 1" on Shaded Scene view)

Notes:
- Reproducible with both GPU and CPU Lightmappers;
- Reproducible only for the HDRI sky; not reproducible for the directional light;
- The workaround is to rebake the lighting with the Shaded Scene view visualization mode enabled.