i've had some trouble with random NPCs and monsters. that when you attack them. they take no damage and also don't respond to you attacking them. infact they just ignore you. then like a light switch being turned on the NPC or monster will suddenly notice you and start reacting to you.
example of this was when i was in the military camp in cordon. while i was attacking it. some of the soilders attacked me like they should. while i had 2 or 3 just stand there doing nothing. i was right next to 1 of these soilders. they did nothing but stick to it's normal scripted patrol route. another soilder just stood in the 1 spot doing nothing.