Zombies not dropping rotten flesh in drowning trap

I built a zombie spawner trap to drown zombies to collect rotten flesh, but the zombies do not drop rotten flesh (or anything else) and nothing is in my collection area. I've watched dozens of them die in the trap, but nothing has dropped. I tried the wiki, Google, the forums and Jira for any reports of anyone else having the same issue but haven't seen anything related to zombies not dropping items when drowned in pocket edition, and the same trap works just fine in the desktop edition (same trap outlined in the "Minecraft Survive & Thrive" youtube series).