Steps to repro:

1. Open attached project and 'BillboardShadowsTest' scene;
2. Observe shadow on tree billboard in Game view.

Expected result:
Shadow on billboard is rendered smoothly without artifacts.

Actual result:
Spherical shadowy artifact on tree billboard (see attached video).

Notes:
- Reproducible in 2017.1.0b5, 5.6.0p4, 5.5.2p1 ;
- Reproducible on OSX 10.12.3 and Windows 10;
- Reproducible only for shadow on Billboard renderer, doesn't repro for shadow on other renderers;
- Disabling shadow on billboard also removes the artifact;
- Reproducible in Editor and Standalone build.