To reproduce:
1) Open the attached scene and load 'ParticleBugTest' scene
2) Play the scene, notice that in camera/game view, half of the screen is drawn white, even though particles are outside camera view
3) Zooming in close the particle spawn area and moving camera around there will bring the same result, even though you did not zoom into same X value as particles spawn point.

It is expected that horizontal billboarded particles are seen as plane by camera rather than a half screen blocker.