Steps to reproduce:

1) Download attached project '2017.3-ExampleProject-ParticleSystemImprovements .zip' and open in Unity
2) Open 'Vortex' scene and select Vortex particle system in the Hierarchy window
3) Observe the Scene view and you will see that transition from positive to negative velocity leads to jagged particle movement

Expected result: Smooth transition between positive to negative velocity
Actual result: Transition between positive to negative velocity leads to jagged particle movement (check attached gif 'JaggedParticleMovements.gif')

Reproduced with:
2017.3.0a2, 2017.3.0a7

Note:
Speed Modifier in the Velocity over Lifetime module was introduced in 2017.3
Unity crashed while opening in 2017.3.0a1