On Farm Street - Ranch Field 2, I'm fighting black sheep for their wool for Duey. Takes three Double Stabs to kill one, and in that time it'll have the animation to do it's magic attack. I kill it as it attacks. I get knocked back and the damage indicator pops up, saying I took about 100 damage, but my health bar doesn't decrease. Correct me if I'm wrong, but I don't think this is an intended feature.
That's normal. It "hit you" after it died, thus the hit doesn't count. It's weird, but as far as I'm aware, that's how it is on every server. Same if you get a lag spike; say in your example you kill the enemy during the lag and after the fatal hit, it bumps into you. You still won't take damage because the mob is already dead.