Power Siphon + Multiple Projectiles not working properly!

I have recently started a witch, using Power Siphon (PS) supported by Multiple Projectiles (MP).

After firing PS and getting a killing blow on a creature, the rest of the projectiles stop doing damage altogether. The creatures getting hit by the other projectiles still display the hit animation, seem to get stunned yet take no damage whatsoever.

I'm currently sitting at 11k DPS, shooting 4 projectiles. Even in normal Mud Flats, I can only kill one mob at a time while spamming my PS.

Note: if I don't kill anything with my shot, all the creatures getting hit by my projectiles take the intended damage. It's only when killing something that the rest of the projectiles seem to get ignored.