While trying out some different texture packs on a large world (303MB), I left the game to go back to the plastic texture pack when I saw that the world was not an option to select from the starting menu. This issue has occurred once in the past, before version 1.8 was released for Bedrock Edition (It happened in August), but the world that was previously lost has reappeared in my worlds menu.
The most recent world was able to be selected, but when I tried to select it, my game said "syncing user data" for a bit too long, so I cancelled it and tried again. When that didn't work, I restarted the game to find that my world had been removed from the list and is effectively deleted. In the previous case, it seemed to be a matter of waiting until my game somehow "found" the world.
The most recent world affected is nowhere on my world select screen, I've searched from top to bottom.
The copy of the world still exists, but the original is gone.