Almost all blocks changed

Almost all of the blocks changed while away from game.

*What I expected to happen was...
I was playing on creative, as usual. When I left the phone, the screen turned itself off, as usual. I expected everything to stay as I had left it, obviously.

*What actually happened was...
When I turned the screen back on, almost all of the blocks around me had changed to something different. This has only happened twice. I had completely different blocks around me each time.

Steps to Reproduce:
1.IDK