Firework Flight Elytra Damage with customised firweork

So
Today I was in my superflat world testing stuff
I took the elytra and launched off with fireworks
Expectation was
No damage without armor on the player
What happened was
Player nearly died by elytra damage
Note:
The type of world used was superflat redstone ready
Player was in Survival mode
Distance from firework was well enough to take no damage
Outcome
Player takes 9 hearts of damage
P.S
Firework was customized