The bug
The Drowned mob has no throwing animation and his hands are just up.
How to reproduce
Run the following command in water:
/summon minecraft:drowned ~ ~ ~ {HandItems:[{id:"minecraft:trident",Count:1b}]}
Set your game mode to Survival
Allow the drowned to attack you
Expected behavior
The drowned would use an animation for throwing the trident.
Observed behavior
The drowned does not use a throwing animation; instead, it simply holds its hands up.
Confirmed for 1.13.1.