The drowned spawn with a trident in 15 % of chance, the drowned who spawnent naturally or with eggs have no problems with this rule.

but the zombies which become drowned(via spawner or other) haven't  trident or nautilus, I tested with 151 zombies which I drowned and none had a trident.

Drowned are supposed to drop on the ground the items of the zombies , but it don't work too