Steps to repro:
1. Open the attached project and 'Repro' scene;
2. In the Lighting window press 'Generate Lighting' window;
3. Wait until the lighting is baked and observe the baked shadow in the Game or Scene view.

Expected result:
The baked shadow is rendered smoothly without artifacts.

Actual result:
The baked shadow has significant square'ish artifacts. See attached screenshots.
Regression introduced in Version 2020.1.0.

Notes:
- Reproducible in Version 2020.2.0a15, 2020.1.0b13 with URP 8.1.0;
- Not reproducible in 2019.4.1f1 with URP 7.3.1;
- Reproducible on Windows 10;
- Reproducible only on Progressive CPU Lightmapper, not reproducible on the GPU Lighthmapper;
- Setting Direct Sigma to 0 fixes the issue, however, in this case, the filtering is not applied;
- Reproducible only with A-Trous filter; not reproducible with Gaussian filter.