When using the start rotation parameter of a particle emitter emitting particles of either Horizontal or Vertical billboard render modes with a pivot offset of any value the particles are scaled and/or flipped.

Between multiples of 90 start rotation the particle is scaled (reaches 0 scale on one axis at 45/135/225/315). At 90 and 270 the particle is 1:1 scale but flipped. This occurs with any pivot offset values. The pivot offset values themselves are still correctly applied.

Repro Steps:

1) Open attached project and open scene "Test"
2) Select object "Vertical Billboard Offset X1" and adjust "Start Rotation" value of the Particle System component.
3) Observe scaling/flipping (Shader is 2 pass: frontface = blue, backface = red)