Stretched Particle with negative parent transform value

Hi, here is my problem, I have a particle system attach to a 2d character. When the character use scaling -1 to change his face's direction, the particle did follow the scaling but its size look different from it original direction.

• The particle use "Stretched Billboard"

• The particle did have scaling mode put to" Hieachy" so it did follow the direction.

• The particle did use "velocity over time" so negative scaling may be affect the velocity which make the effect look different for each direction.

=>As you can see in this image:

"A" was original direction,

"B" was -1 direction

Do you have any idea to make those look the same for both direction without change "Stretched Billboard" render mode?