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)
This appears to be a duplicate of MC-169174. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed. Like I said earlier, it just gives the team a general understanding.