Drowned Do Not Drop Nautilus Shells

I have been trying to farm for nautilus shells in order to create a conduit. . . From what I understand, drowned are always supposed to drop whatever item is in the offhand slot; however, I killed give or take 5 drowned (all with nautilus shells) with a looting three sword, and a trident (mainly to see if the results would be different). Either way, only rotten flesh dropped after the drowned were killed.