A single player world I was playing on as recently as this morning before I got home from school has suddenly just upright disappeared. It's still in my saves folder, but it doesn't appear in my selection menu.

When messing with this, I noticed that the "Parkour Map" save, if removed from the saves folder, will update when exiting and reentering the selection menu. I tried the same with the other world but to no avail.

I also tried duplicating my save file (not present on the screenshot I took) but that did nothing either. It has something to do with the save file itself that the game refuses to cooperate with.

Update:
I did some quick research to see if I could find a parallel, and the bug report MCPE-40963 seems to be described similarly. There isn't that much information on their post, but it says 1.8. So this might be happening in multiple versions. Just thought I'd add that.