When launching a firework rocket (vertical or horizontal) at a mob (either manually, with dispensers, or with a crossbow), there will be times where the mob takes damage when the explosion occurs far away from it and other times where it takes no damage even though the explosion occurs right next to him.

It's probably a mismatch with the particles and the actual explosion.

Steps to Reproduce:

• Make a block pillar ~12 blocks tall.
• Make a redstone clock (not so fast) to activate a dispenser right next to the base of the pillar (facing up).
• Fill the dispenser with fireworks of duration 1 and activate the clock.
• Put a Villager (or any other mob) on top of the pillar and watch for a while. (1)
• Put a block above the dispenser and above the level of the pillar so that it makes the fireworks stop just at the height of the head of the villager (or mob) and watch for a while. (2)

Observed Results:

(1) The villager takes damage when the explosion (or at least the particles) are far below his height, and takes no damage when the explosion happens right next to him.

(2) Rockets that stop at the block explode next to the villager, dealing damage.

Expected Results:

(1) When the explosion (signaled by the particles) occurs right next to the villager it should take damage.

(2) This part works as it should (and shows how 1 should work) since the particles indicate the correct place of the explosion and the damage is applied as expected.