**
Samsung Galaxy S4
Android 5.0.1
(also playing with a moga controller, probably has nothing to do with it)
**

I've posted a detailed thread about this on reddit:
https://www.reddit.com/r/MCPE/comments/4fn0ni/realms_bug_some_items_duplicate_some_disappear/

Someone in the comments found me the way to replicate the issue.
Basicly whenever you have an item in say like, the 4th slot of your hotbar,
and you just press the "use" button (L1 on my controller), it will duplicate whatever is in that slot into your inventory.

This is a pretty accurate way to replicate it, easy enough.
For some reason however I can't get it to always work immediately, only when I swap around the places in the hotbar again and again, but you can in fact keep duplicating stuff. Tested this on a full stack of diamond.
Whenever I would take a load of food with me underground, I'd find that after a while the food disappeared, and that the slot it was in, is now replaced by a duplicated stack of cobblestone.. that kind of sorcery.

Read the thread if you like, it has a lot more details.
Anyway, this is pretty game breaking, hope it gets fixed ASAP, it's the only thing annoying me on the MCPE realms.

Good luck!