In 1.18 experimental the Stealth Kill animation will often play 90 degrees offset to the left of the zombie you are targeting. Also frequently it happens out of sync, so you first do a regular slash but immediately after an offset stealth stab killing the zombie. So the zombie grunts and occasionally gets his aggro scream off before dying. When that happens the zombie will keep growling a while after death.
So there is some desync and offset problems in general with the animation. I will do some more testing and grab some video when I have time.