If you try to launch a firework with a dispenser and flight:3 the hight that the firework is flying is like flight:1 when you start a firework from the ground. Flight:2 and flight:1 are very similar to flight:3. It only habbens with dispensers and not when starting a firework with a right click or a /summon command.

This affects the version 1.15 and all versions above.

(Sorry for my bad english)