I am not certain whether this is a bug.
All I can say is that prior to 17w47a, the NBT-tag "Fireworks" matched the item name "fireworks". This tag specifies details about the item like "Flight". In the scope of The Flattening, the item got renamed to "firework_rocket", but the tag still remains "Fireworks".
The tag should be adapted to the new name as it contains additional information about it.

Edit: With "adapted" I mean:

firework_rocket{FireworkRocket:{}}

There might be other cases where this rename did not happen, but I do not know where to look for.