Only a couple items can be placed in the off hand with fireworks being one of them (i assume cause of how they work with elytras) yet when put in the off hand they will be impossible to be launched from the ground or used when flying an elytra.
Linked issues
duplicates 1
Comments 2
This is not a bug, see MCPE-49356.
Im not expert at Minecraft but i can tell you that the fireworks are'nt in the off hand for the elytras.
They are there because you can shot them with the crossbow.
That is a ability to make the crossbow shot fireworks instead of arrows on your inventory.