I played as Zombie Brand recently on Proving Grounds on the live server (not PBE) and when I was basic attacking, whenever Brand would be near the point when the projectile would launch, he instantly jerked back to a previous state in the animation and then followed through with the rest of the animation. This bug repeated over and over.

I don't think adding attack speed affects it. I had an Ezreal on our team and after he W'd me late into the match, I noticed the bug still occurred. Because of this, I would guess the animation was (accidentally?) sped up in the recent patch to finish faster than he's allowed to attack, so it forces the animation to go back.

I would have made a video, but I don't have any means of recording one.