Steps to repro:
1. Download the attached project and open the 'BeachCoast' scene;
2. In the Lighting window press 'Generate Lighting' button;
3. Wait until the Lighting is baked and observe the scene.

Expected result:
The baked lighting from area lights is smooth and doesn't contain visual artifacts.

Actual result:
Firefly artifacts all around the race track floor produced by the baked area lights. See attached screenshots.

Notes:
- Reproducible in 2020.2.0a10, 2019.3.7f1;
- Reproducible on Windows with GeForce 2080 RTX;
- Reproducible when baking only with GPU Lightmapper; not reproducible with CPU Lightmapper.