To reproduce:

1. Open repro project
2. Build and run scene for WebGL

Actual vs Expected: there should not be any 'shadow' artefact on plane

Reproduced: 5.6.3p4, 2017.1.1p4, 2017.2.0f3, 2017.3.0b7, 2018.1.0a4 (Firefox and Chrome only)

Notes:
- Not reproducible on plane created with editor
- Not reproducible on IE or Safari
- The issue is related to realtime lighting only

Also reproducible on Android --> HTC 10 - Adreno530(GPU), both on OpenGLES2 and OpenGLES3

By design: fog mode is set to exponential, while the geometry does not have enough vertices for smooth exponential change.
Solution: Either provide more vertices or set fog mode to linear.