Schofield's Second Law of Computing states that data doesn't really exist unless you have at least two copies of it. In any case, this is why you make regular backups of any data that is important to you, otherwise, it may as well not even exist - I can't even imagine losing my first world, which will be 8 years old in a few months, or all of the mods I've worked on for almost as long (thousands and thousands of hours of work in total): MC-161823 Chunks can occasionally be misplaced upon loading a world Otherwise, the game has a serious world corruption issue which causes chunks to randomly move around, which can even happen for no obvious reason at all (you play like usual then the next time you open the world it is corrupted): This most likely happened because you opened the world in an older version, hence the 'unknown update' warning, as the game only recognizes worlds last loaded in itself or versions older than itself (this message can also occur on worlds from before 1.9, when save versioning was added always check with the latest version to be sure that it isn't from a newer version though), and you should absolutely never downgrade a world, which can even result in total destruction of everything in loaded chunks (the game will completely regenerate them from scratch and only your inventory/player data will survive, possibly not even that if you downgrade far enough).

Lunar client update