Not sure whether this is intended or not.
To reproduce, spawn a bunch of zombies in water. Note that some of them will magically summon tridents, and sometimes fishing rods out of nowhere when they transform into drowned.
Linked issues
Comments 4
Makes me wonder if zombies with items/gear lose their stuff when they transition to a drowned zombie.
I tested it yesterday, zombies holding any item, whether they spawn with it or not, drop the item 100% of the time when turning into a Drown.
This can be used to get the item a zombie is holding with 100% certainty it will drop it.
see MC-127291
In Zombie or Drowned form?