I recently noticed a major bug in the Android version of Minecraft. Gameplay seems to still be going even when your device is off and you're playing singleplayer, unless you quit before you turn it off. I've noticed this with furnaces and hostile mobs. I even rage-quit a world after being killed when there was nothing around when I was last playing (I didn't even see it happen; I just saw a "You died!" right after unlocking my phone).
Here's a simple way you can reproduce this problem:
1. Place a bunch of stuff in a furnace, including fuel & smelting ingredients
2. Turn off your screen and wait a couple minutes.
3. After turning it back on, you'll notice that a lot of smelting has happened during the time your screen was off