He,
I don't know if its a vanilla bug because i play only btw...
I have meet this bug sometimes with official launcher , custom launcher , old computer or new computer.
I launch a lot of world or lot of new world and sometimes minecraft lost my world completly ! I think there are a problem with a variable !
Look at this picture :
Minecraft dont show my world name : " ko teknik "
And the only way to get back my world its to create a copy of the folder world " ko teknik " and after create a new world inside minecraft with exactly the same name for exemple " ko teknik " after the world launch close minecraft, and put the files back from your copy....
It happen to me maybe 10 - 15 times ( but i play btw a lot ) its not a huge probleme for me because i know how to get back a world ! But for casual or new player it will be very frustating to lost a world !
Anyone have meet this bug ?