Steps to reproduce:
1. Make sure that you have WebGL platform support installed
2. Download and open attached project (New Unity Project.zip) and open SampleScene
3. Open the project in a 2019.4 LTS editor version
4. Observe the appearance of the Shaded view, and the Lightmap-0_comp_light file
5. Now open the project in any other editor version that is 2020.1.0f1 or newer
6. Notice the drop in quality both in the Shaded view, and the Lightmap-0_comp_light file
Notes:
- Reproduces in: 2020.1.0a15, 2020.1.0a17, 2020.1.0a22, 2020.1.0a26, 2020.1.0f1
- Does not reproduce in: 2019.4.38f1, 2020.1.0a1, 2020.1.0a13, 2020.1.0a14
- This issue was introduced in: 2020.1.0a15
- Prior to 2020.1.0a15, WebGL used DXT compression by default
- Overriding to DXT compression manually after 2020.1.0a15 results in quality reduction
- Post-2020.1.0a15 release WebGL uses RGB9E5 compression by default, but users cannot override to it manually: https://fogbugz.unity3d.com/f/cases/1424639/