Steps to repro:
1. Open the attached project and 'Lights and Shadows' scene;
2. In the Lighting window press Generate Lighting button;
3. Wait for the lighting to bake.

Expected result:
No artifacts are rendered on the plane with baked area light.

Actual result:
Black directionality artifacts are rendered on the plane with baked area light. See attached screenshots.

Notes:
- Reproducible in 2020.1.0a20, 2019.3.0f5;
- Reproducible on Windows 10;
- Could not test in 2019.2.19f1 or earlier since all the geometry is hot pink after opening the project;
- Reproducible with both CPU and GPU Progressive lightmappers;
- Reproducible when AA samples is set to 4 and higher; not reproducible with AA samples set to 1;