The bug

With commands you can load a crossbow with entities other than arrows (such as a trident), however the crossbow will launch a arrow, not that entity.

In case you loaded a trident into the crossbow, you can see that the projectile is a trident in the crossbow's tooltip.

How to reproduce

1. Paste this command into a command block or the chat with cheats activated

   /give @p minecraft:crossbow{ChargedProjectiles:[{id:"minecraft:trident",Count:1b}],Charged:1b}
   

2. Activate the command block or run the command
3. Hold the crossbow and fire it (Notice the word "Trident" in the tool-tips)
   → The projectile fired will be a arrow instead of a trident