Steps to repro:
1. Pull the Sponza URP project from here: https://ono.unity3d.com/test-data/gi-tests/SponzaLWRP and sync to revision 1ba7fdd9f457acff81428824ec6f3460e3e72ab4;
2. Open SponzaLightingDay scene;
3. Pull the Sponza scene to the Hierarchy;
4. Make sure that the scene is selected as a master scene, if not, double-click on SponzaLightingDay scene in the Hierarchy;
5. Make sure that GPU Lightmapper is selected as a baking backend for the SponzaLightingDay scene;
6. In the Lighting window press Generate Lighting button;
7. Once the bake is done, navigate to the Lightmap visualization view mode.
8. Set tonemap to -4, take a screenshot.
9. Restart the editor.
10. Observe the difference in brightness in the tonemapped lightmap scene view mode.
11. set tonemap value to -4.1, brigness changes a lot
12. set tonemap value back to -4, things now look as expected.

Expected result:
Tonemapped lightmap should be quite dark.

Actual result:
Tonemapped lightmap is too light.

Notes:
- Observed on version: 2021.1.0a1.218 (trunk rev c3eb06b4e364).
- Does not reproduce in: 2020.1.5f1, 2020.2.0a14, 2020.2.0a17, 2020.2.0a18
- Reproduces in: 2020.2.0b3, 2020.2.0a19
- Affects all render pipelines (BRP, URP, HDRP)
- Affects both the CPU and the GPU lightmapper