Converted Drowned don't drop Drowned loot.

Apologies if this is a duplicate, but I couldn't find an entry exactly discussing my problem. I built a Drowned farm utilizing a Zombie Spawn Cube that funnels zombies into water trap and converts them into Drowned, allowing me to kill them. I have killed about 500 with a Looting 3 sword and they only seem to drop loot from the Zombie loot table. Is this an intentional feature in the 1.13 full release, or is this a bug?