When trying to uppercut a zombie after a punch, the first punch acts as a heavy hit even though it's a normal hit and blows the zombie back resulting in you missing the uppercut.

Hit RT to regularly hit a zombie & then immediately hit Y to trigger a uppercut. You'll notice the zombie acts as if the regular punch is a heavy hit and blows back causing you to miss the uppercut rendering the uppercut or any heavy hit chains almost impossible unless the zombies is backed into a wall.

I believe melee combat should revert back to the previous version where if you attack the zombie in the head, it interrupts their attack. In its current state, you can be hit while hitting the zombie.