I have found that if you double tap the home button when on a world, or leave the game while playing without clicking "Save & Exit" the world itself saves, but the player inventory, such as exp and items/armour does not save. Then when you load the world up after exiting, the world remains as it was when you left. For example, I found a mob spawner area with two chests, I took the items out, then switched to a different app real quick, then back to minecraft. When I got back into my minecraft world the items from the chests that I previously looted were not in my inventory but also were missing from the chests and surrounding area, as if they had vanished. I'm not sure what is causing this but it has happened 3 times.