There are 2 reasons he would stand up early, either the DA is very low, so you do too much damage too fast, or, you accidentally knife his exposed chest eyes through his torso. Any damage with the knife to the eyes in front will make him start standing up. It's way easier to get this meme on Leon because his knife slash goes a bit deeper. You avoid this by either backing up a bit, or staring straight at the ground while knifing his back. DA for this fight with most recent strats should be around 6800 on CAS and LAS.

Edit: Never mind the DA thing actually, after testing it I'm certain it's just the slashing of the chest eyes that triggers him to get up.

Go to your game files and delete the re2_config file. You will have to change your keybinds, but it should fix the stuttering problem.