Can load fireworks with the crossbow in the offhand while holding a firework rocket in the mainhand

Probably intentional. You can load a firework rocket with a crossbow in the opposite of loading a crossbow in the mainhand with a firework rocket in the offhand. This is only for the fireworks in the mainhand, which shouldn't apply to arrows. It is shown during Minecon 2018 see here.

How to Reproduce

1. Get crossbow and firework rocket
2. Press F to swap items in main and offhand
3. Load the crossbow in the offhand
   → The crossbow is loaded with a firework rocket, even if the crossbow isn't in the mainhand.