I'm sure this is just an oversight - loyalty-enchanted tridents will drop themselves as an item if they're attempting to return to a player who is dead, and setting doEntityDrops to false doesn't prevent this from happening.

Steps to recreate

-Run command /gamerule doEntityDrops false

-Give yourself a trident with a Loyalty enchantment

-Throw it, then kill yourself before it can return to you

-The trident will be found sitting on the ground as an item.