this doesn't work on other mobs except zombies, husks, and pigmen, and other sources of damage knock back zombies. when you hit a zombie with a sword or your hands, it randomly ignores knock back and continues to move forward.
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it does happen when they are on fire but it also happens randomly when you hurt zombies, try killing 20 hostile zombies with a sword and they will sometimes completely ignore knock back. (unless it is a sweep attack)
I actually tested it out in a villager house, the zombie wasn't taking any fire damage, and i hit the zombie, after the tenth time it didn't respond to the knock back and the zombie dealt damage to me, it doesn't seem to be affected on hard mode either and it seems to happen completely randomly in any case. and a skeleton shot a zombie and it happened too. I think the zombie damage knock back AI is broken.
I'm pretty sure this is WAI. Zombie's & Zombie Pigmen can spawn with knockback resistance.
It kind of makes sense anyway, or else zombies would be way too easy to kill, and it also makes sense why a hostile zombie would ignore "stagger" anyway.
Is this during the day, when they are on fire?