Pistons don't always push mobs over the edge

Pistons pushing don't always push mobs over the edge.
There is a world download in the attachment/see screenshots for how to recreate.

Probably a lot of mobtraps will be broken by this bug. I am surprised nobody found it yet (couldn't find a issue here)

Edit:
Doesn't have anything to do with the direction, recreated the bug with different levers placement and it didn't pushed them of in different directions.(two north and two west) (see new world download)