The new FireworksRocketEntity for fireworks don't save all of their properties to the Anvil Entity NBT list; this makes them non-persistent across world closing and loading. There is an additional issue that since there are no tags to use, Spawners that spawn fireworks don't work properly. When spawned from spawners, FireworksRocketEntities and their accompanying particles exist briefly before despawning, preventing FireworksRocketEntity spawners from working.