I read that in Java edition that is the case, however in bedrock edition both naturally occurring and converted drowned will have the same drops. I read this on the minecraft wiki, and it had always worked for me. I have a farm setup that funnels zombies from a dungeon spawner and into a water filled column for conversion to drowned with hoppers as the floor to collect all of the loot.
For the past several months I have been able to farm gold ingots, tridents, and shells easily using this method. It was not until the bedrock 1.10 update that they quit dropping the listed items. I also re-read the change log and I never found anything related to changes for converted drowned drops. I could be missing something, although I know for the past several months it has worked fine.
Windows 10 player here. I am also experiencing the same bug. After farming over 1000 zombies converted into drowned I had no rotten flesh, gold ingots, nautilus shells, or tridents.
I then went to a creative test world and farmed an additional 200 converted drowned with a looting 3 sword and found that I still had not received any rotten flesh, tridents, gold ingots, or shells.
After that I spawned in 200 drowned with a spawn egg, then farmed them with a looting 3 sword again and I received a normal amount of the regular drowned drops.
I rely heavily on my drown conversion farm and it would really be appreciated if this was fixed.
After multiple different tests on Minecraft windows 10 edition version 1.10 I can also confirm that converted drowned drop nothing, and that naturally spawned drowned seem to be fine.
After farming nearly 1000 converted drowned from my drowned farm on my Bedrock dedicated server, the only drop I receive was the armor they are wearing before they are converted. I confirmed this in a test creative world as well and I got the same results.