Taking any type of firework rocket and renaming it (e.g.:missile) will result in the firework item rendering its new name when changing the inhand item to the firework rocket, but not when looking at the renamed firework in the inventory. Instead, it displays its original name, "Firework Rocket"

Note: firework NBT (crafted with firework star) included:
-flight duration: 1
-trail
-purple
-large ball
-fade to black

(additionally note that I had toggled my inventory cursor off at the time)