Worlds not saving!

This bug has been happening to me for a while now. After playing for about thirty minutes I decided to hop on to another app for a sec. When I went back to Minecraft, all my progress was gone!

Note, the many times that I've had this glitch have happened while I was playing alone and with about three more friends in the world. This glitch is extremely important because I've noticed it is affecting many other players and has been going on for longer than you think.

Just to let you know, after a few consecutive incidents of my worlds not saving, I've tried to press 'save and quit' after a while of playing to save my progress, but half of the time Minecraft will freeze and proceed to my home screen. When I go back on, it will not have saved.

One more thing I think you should know, now that there's an image next to every world of what you saw last before exiting it, I've noticed something weird too. Before I went into a specific world with my friends, the image was just grass plains because we hadn't built anything yet. When I decided to press save and quit for the sake of what we built, my screen froze, exited the game, and didn't save. When I went back to Minecraft, though, the image for the world was of what we had just built so it seemed like it saved, but when I entered it again, nothing was there.

For the pictures I added, you'll see the first ones don't have anything next to the birch wood building, but the last two do. The last two show what I saw after exiting out, and the one with just the birch building show that the image was actually wrong and there was no dark wood building that saved.

Please fix this! It's taking the fun out of playing.