After you switch Renderer mode from Mesh to Horizontal or Vertical billboard in Playmode artifacts appear.

To repro:
1. Create a new Project.
2. Add new Particle system.
3. In Inspector Check Renderer -> Set Renderer mode to Mesh.
4. Press Play.
5. In Rendered switch Renderer mode to Horizontal or Vertical billboard.

Actual result: Artifacts appear, that stay even if you switch to other Renderer mode (see disco-particles.gif below).
Expected result: Horizontal or Vertical billboard is shown (see HorizBillboard_5.5.a5..gif below).

Regression since 5.6.0b1.
Works as expected in 5.6.0a5.