I think they just haven't added the drowned loot table.
This is a duplicate of MCPE-31525
I can confirm this on windows ten.
this is also happening on windows 10.
i rebooted but the items are still not there, and sometimes random stuff disappears when i change tabs. no matter if you can "reboot" your game to fix it, the bug shouldn't be in the game in the first place. i think the reason why there are lots of bugs is that Microsoft is doing the same thing with minecraft as they did with halo, they force the devs to work faster and pump out more and more updates/games and put deadlines, that the games are full of bugs. i understand Mojang.
How about just make the growing of a tree not even have "fallen tree" as an option. This is very annoying for tree farms in that you dont get the sapplings. That is all i have to talk about for the bug. Now what would be cool if there were fallen trees with leaves still on them.
Did you load a bunch of chunks/travel alot?
this also happens when you are on your horse when you exit the game.
Make sure you have turned off all textures before reporting a bug.