My friends World has disappeared and will not show up in his single-player worlds yet it is still in the saves folder in .minecraft. After looking to see if this was a problem I found issue MC-193198 which has the same problem where their world is gone but my friend's level.dat file has no change to it. He sent me the world file and everything seems normal but the world doesn't show up in my single-player worlds either.

WORLD FILE : https://www.mediafire.com/file/0hntymbvc9ase7j/file