The issue that occurs from my observation is that the result given by the CraftItemEvent is missing its NBT data that is shown in the crafting table. When I print out the NBT of the result item stack it contains no NBT. 

In the following example I am using the Firework itemstack with different powers however the event seems to not pass that NBT information along. As seen in the images the result has 1 NBT tag however the event "getRecipe().getResult()" is reutrning a plain firework with no metadata.