How to reproduce:
1. Download attached project files and open "Scene" Scene
2. Enter Play Mode
3. Change Pont Light Transform Y position from 1 to -1

Expected result: "Horizontal Billboard" Render Mode in the Renderer module of a Particle System the normals should be facing up
Actual result:"Horizontal Billboard" Render Mode in the Renderer module of a Particle System the normals are not facing up. While changing Point Light Transform Y position, "Horizontal Billboard" appears bright at minus position and dark at the positive position

Reproduced with: 5.5.4p3, 5.6.1f1, 2017.1.0p4, 2017.2.0b8, 2017.3.0a5