When a firework rocket is launched and you observe its launch from above/below you will see the rocket flying up sideways. In other words, it seems to always be rendered as a sprite.

Fixing the orientation of the graphic would fix the bug but the rocket might then become hard to see from above/below. Changing the sprite to a 3d object would fix both issues.

(This problem is slightly similar to the flames of torches being displayed slanted when lying in a bed (as reported in MC-926)