To reproduce, collect firework rockets, put them in your inventory or hotbar, (make sure they're prioritized over arrows if you have arrows in your inventory) and the crossbow loads a firework rocket instead of an arrow. It did not work like this in earlier 1.14 snapshots.
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Comments 3
I think this is intended, because bows will use the nearest shoot-able projectile slot in the inventory
I don't think it's intended. It didn't work like this earlier in the snapshots such as 18w43a. Also the Minecon livestream showed crossbows shooting fireworks from the offhand (not the hotbar since that is likely not supposed to work): See https://www.youtube.com/watch?v=HoMDyRqMNMA at 16:52
This could be an annoying problem when trying to shoot mobs or players while flying using elytra.