While almost every enemy is affected by the stun effect of the firecrackers, one of the few enemies that are immune to the stun and only their posture is affected are the headless and headless apes. Now, if you've played Sekiro, I bet you were as shocked as I was fighting the ape the first time, but him not getting affected by the firecrackers even though he is a beast is just what I would expect from the game, to hit you with a curve ball when you least expect it. Nonetheless, it would be best if you welcomed the challenge; after all, it is a FromSoftware game.