When in a superflat world (gamemode doesn't matter), trying to boost elytra flight with fireworks results in the fireworks going off like normal. I am not aiming at a block, because the fireworks start from my position and go up like normal. This only happens when I am actually flying with the elytra, otherwise nothing will happen.

EDIT: this only seems to happen when this is the first world you join after starting Minecraft, and only if it's a superflat world? really weird

Possible reproduction:
1) Start Minecraft (restart it if it's already running)
2) Create a new creative superflat world (preset does not matter)
3) Get elytra and some fireworks
4) Try to use firework boosting

Note that it works after reloading the world, or if any other world was loaded before.