The bug

Drowned are able to pick up items and weapons, but do not recognize tridents as a weapon and so only pick one up if not holding an item already.

Steps to reproduce

1. Summon a drowned that can pick up items

   /summon minecraft:drowned ~ ~ ~ {CanPickUpLoot:1b}

2. Give it a trident
   → The drowned will pickup the trident
3. Give it any sword, such as a wooden sword
   → The drowned will prefer the sword over the trident even though it does less attack damage