Does stretched billboard conflict with Velocity over Lifetime in particle module?

Hi all:
I want to create a stretched billboard particle with Velocity over Lifetime enable. So I make a test and get very strange result. It seems the stretched billboard particle will always flip around its pivot when the velocity lower than a value.Here is the step to create a test particle:
1.Create a stretched billboard particle.


2.Enable Velocity over Lifetime, set X velocity to linear decreasing and Y and Z velocity to 0.

3.The result.Particle begin to flip left and right around its pivot in about 3.98 second.

I check all settings and still get the same result. Is this a bug or I get something wrong?
And help will be appreciated.Thank you!


When I test with X velocity set to line increasing, ervrything is wonderful. So what’s the problem?