Drowned that were originally zombies do not drop any items when killed. Not just tridents nautilus shells and the like... they dint even drop rotten flesh. Naturally spawned Drowned do however. I suspect what is happening is when a zombie drowns the new mob has no loot table associated with it. Ive killed 100s of drowned in a spawner grinder that flushes them into a pool to drown them first and can comfirm that not a single one drops anything whatsoever.