The Bug:
Firework explosions do not render when they occur over 64 blocks horizontally away from players. Given that fireworks are meant to be seen from distance and can be shot by a player further than 64 blocks away, I believe this to be a bug.

Steps to reproduce:

1. Fill a dispenser with exploding fireworks and create a redstone line that will activate it more than 64 blocks long

2. Observe when dispenser is activated and the player is within 64 blocks horizontally of the dispenser, dispenser shoots the firework and you can see the firework explode

3. Observe when you're beyond 64 blocks horizontally the dispenser will still shoot the firework BUT you don't see the firework explode

Videos:

Video showing 64 block rendering bug: https://www.youtube.com/watch?v=og2gY7hI77c
Flight Duration 3 Fireworks shot via Crossbow affected by the same bug: https://www.youtube.com/watch?v=JIa5z0iNmkQ