The ability to break free while subject to a knockback animation has been removed in many circumstances. You are still charged the cost of the break free, but the break free (and the immunity derived from it) do not actually occur. You must wait until the knockback animation is completed to actually break free.

This tends to happen most often when the knockback animation interupts/cancels an animation you were half way done with.

The best example is when you get knocked back while rooted (and still going through the root animation). There is no counter.

Many players playing in duos have already discovered this and are timing their roots / KBs to happen at the same time