Steps to repro:
1. Download Sponza URP project: https://ono.unity3d.com/test-data/gi-tests/SponzaLWRP
2. Open 'Sponza' scene;
3. Drag 'SponzaLightingDay' scene to the Hierarchy;
4. In the Hierarchy double-click on 'SponzaLightingDay' scene to make it active;
5. In the Lighting window press 'Generate Lighting' button.

Actual result:
Dark artifacts are shown on the baked scene's geometry. See attached screenshot.

Expected result:
The lightmap on the baked geometry should be smooth and not contain such type of artifacts.

Workaround is to set the Max Lighmap Size to 1024 or lower and rebake GI.

Notes:
- Reproducible in Version 2020.1.0a9, 2019.3.0b8, 2019.2.10f1;
- Not reproducible in 2019.1.14f1 (Optix denoiser for GPU PLM wasn't introduced in it);
- Reproducible on Windows 10 with NVIDIA GeForce 2080RTX GPU with the latest NVIDIA drivers;
- The issue affects both CPU and GPU progressive lightmappers;
- Reproducible with NVIDIA 441.41 and 436.48 drivers; not reproducible with the NVIDIA 430.86 driver.