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.
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Can someone explain this a bit better please? (Y'know, with the steps to recreate this problem etc)
This ticket has been marked as a duplicate of MC-4688 , in favor of the better description/information provided.
This happens with any NBT edit, not just fireworks
(such as health changes, item changes, etc)