On two occasions now the worlds I've started have disappeared from my singleplayer menu while still appearing in the saves folder of my .minecraft files. I believe I've closed the game properly and not just shut off the computer with it running or anything, so I'm not sure if this is an accident on my part or a bug, though I've never had issues with this before. Thanks so much for the help!

EDIT: When going through my remaining worlds to find one to play, I loaded the "RIP My Other World" world and after checking which builds are in the world, I left to change the world's Icon, then reloaded the game. After I left that game to check another world, I arrived back at the singleplayer menu only to see the RIP world had disappeared as well. I clicked on another remaining world (one of the 4 cities ones) and went to the edit world menu, checked some things, and left that screen to go back to the singleplayer one. THEN I saw the RIP world had reappeared? I took screenshots of the world after it disappeared and reappeared. The other world, however, hasn't returned. I know for a fact the game saved and closed as it normally should, so I'm less sure it was an accident on my part that caused it?

(Also, the Forever Survival worlds was deleted intentionally so it disappearing wasn't a bug- Sorry if my wording on any of this doesn't make sense!)