Steps to repro:

1. Open attached project and 'SampleScene' scene; on OSX;
2. Let the scene's lighting auto bake;
3. Observe the plane.

Expected result:
Shadows casted from cubes are rendered.

Actual result:
Shadows casted by cubes are not rendered on the plane. Artifacts are shown in the Scene view when moving the viewport.

Notes:
- Reproducible in 2019.2.0b6;
- Reproducible only on OSX 10.14.5, not reproducible on Windows 10.
- Reproducible only for the shadows casted from/to probe-lit GI contributors; not reproducible for shadows casted from/to dynamic objects;
- Reproducible only for the Mixed lighting (shadowmask and subtractive); not reproducible for Baked Indirect;
- Not reproducible for Realtime GI;
- Not reproducible for Baked GI.