When you put a firework rocket in an enchanting table with a flight duration level or an NBT tag the game forgets it and the rocket is no longer stackable with the rest and resets to flight duration 1 (this only happens when shift clicking, dragging it in the place doesnt make it lose its nbt data)
Thank you for your report!
We're actually already tracking this issue in MC-17431, so I resolved and linked this ticket as a duplicate.
However, that ticket has already been resolved as Fixed. The fix will arrive in the next version or is already included in the latest development version of the game.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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