Snowballs which have been thrown through a nether portal do not apply knockback to mobs they collide with on the other side.

Snowballs thrown through nether portals DO trigger the "hurt" sound effect and flash of red, and apply damage (if the target is a blaze), but no knockback.

Snowballs thrown in the same dimension as the target mob apply knockback as normal.

Steps to reproduce: 

1) set up snowballs to be thrown through a nether portal (easiest method(overworld): snow golem on one side of nether portal, zombie on other side of nether portal)

2) travel to nether, toss an item through nether portal to load overworld chunks (thus triggering the snowballs to travel to the nether)

3) have the snowballs hit a mob in the nether