Make fireworks of any type.
Shift click the fireworks to move them to your hotbar. (Fireworks will be sent to the offhand)
Attempt to use fireworks on the ground, as a boost for elytra, any use.
Usage will fail because fireworks cant be used from the offhand.
Shift click the fireworks in your inventory, it will move from the offhand slot to the hotbar.
Attempt to use fireworks on the ground, as a boost for elytra, any use. 
Fireworks will function normally/ as expected.

Either disable the offhand function for the fireworks or make them usable from the offhand.
Java has so many items that can be used from the offhand and Bedrock has next to 0 by comparison.. Even the items that can be placed in the offhand do not function in that position.

For parity, this should be fixed to match Java.

I tried to attach a short video clip to demonstrate this issue but the shortest clip I could make still exceeds the minute file size limit....