Worlds dissapearing but still in files and unpredictable behaviour when making world with same name

Affected Versions: 1.14 snapshots, 1.14, 1.13.2, 1.13.1, 1.13, 1.12.2, 1.12.1, 1.12 etc

ACTUALLY NOT RESOLVED, MC-315 is not a duplicate because that is a chunk bug when this is the file/world disappearing completely, so ignore "Resolved" and ignore "Duplicates". Whoever put that there, please remove it immediately so people are not misled.

When I make a world sometimes it randomly disappeared, this only happened twice and both times I got varying results.

1st result: The world was still in the files and I backed it up, but the backup wouldn't work either, when I tried to create a world with the same name it said "Will be saved in [WORLD NAME HERE]-".

2nd result: Unlike result 1, it disappeared from the files completely and when I made a world with the same name, it worked but when I logged in, it had a completely different seed but the scoreboard I made was still there and worked fine... Later, I tried to log in to the world and it just didn't work and kept on crashing but it was ONLY THIS WORLD...

How to reproduce:

? (I don't know how to reproduce it at all)

Please comment if you had a similar scenario happen to you, thanks!