I have been playing Lavonia experimental. have noticed many occasions that when melee attacking zombies with power attack or just normal combat often, they die as though it was a sneak attack.

during combat, several of their attacks don't cause damage when, to my understanding I shouldn't be able to block. Usually, one-handed attack followed by their power attack. I notice this happens most if my characters backed against an object or wall unable to back up, while holding the block button and attack at the same time. This isn't always but it is common enough you should see it within a few fights.
Also, I should say that just holding block and not backing up (not blocking) but holding block and attack without an object obstructing my backwards movement does the same. I am not sure how often that happens though