My Minecraft world isn't saving anymore

My survival world was normal until today. I have no idea why it has stopped saving progress. I thought it could be some bug with the Nether, with the horses, or something like that. But I have done A LOT of tests and I noticed something very strange. ONLY THIS WORLD has this issue. Other are still saving.
I save the world by pressing the "Back to Menu" button, without closing the app or turning off the device.

Can someone PLEASE help me???? I can't play on that world anymore, it shows the proper screenshot, but when I enter, I'm back to the old save, with all my old stuff, old XP and overall, old world...

(By the way, sorry if I wrote something wrong, I'm trying my best to speak english correctly)

I play on a Samsung Galaxy J5 2016 (SM-J510MN)