When loading a superflat world from a previous beta that had caves and cliffs experimental features turned on, any previously created chunks have been deleted.
It appears that the old world has been shifted down 64 blocks into the void, which means the lower 64 blocks of the old world have just been deleted.
Update 1.17.40.21:
While the world now loads old chunks, all block data hasn't been shifted down properly, all signs, chests, lecterns, etc. have lost their data.
Steps To Reproduce:
Load any super flat world that had caves and cliffs turned on
The world will have several deleted chunks
Expected Behaviour:
Superflat worlds don't get deleted
Observed Results:
Superflat world chunks from old betas get deleted
Related issues
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Confirmed. I will attach my world. It was not open at 1.17.30.24
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The same thing happened to me on my realm. Chunks just dissapeared.
Also, I never used beta, so....

Any way to fix this? I've had 3 years progress on a massive city and to see it go will be deviating

I have the same problen i had so much progress and now it is turned into a giant hole in the ground
Is there a way to fix this it is very bad and i was planning to play with a friend guess thats not happening

ive worked hard my city world and when i came to minecaft after 1.17.30. this happend and i was sad because the church i made was deleted from this bug and i hope this is fixed because i worked hard on this city world pls and thx.
i will have updates on this soon
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Its happened to me aswell. All working perfectly fine before the update. Updated to 1.17.30 and poof, my world gets wiped.
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Is there a way to fix this? It just happened to me and I lost my entire quarantine project.
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I built a huge fleet of vehicles and buildings in my super flat world, but since I updated 1.17.30.23, the world has become like this. . . I am very sad now, I hope this bug can be fixed as soon as possible, so as not to let more people be affected.
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I have the same problem in Win10:
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Confirmed here also. Caves and cliffs was activated prior to 1.17.30 update and after the whole map was shifted down 63 blocks in the y coordinate.
I have a team of builders on hold waiting to resume the month of work they already put into this custom world painter map, we are anxious to hear about any progress to fix this issue.

Playing on a custom map converted from Java. It is technically listed as suoerflat but it's not flat. Whole server is down due to this bug when can we expect a fix.

I spent hundreds of hours on a pixel art and city world with my friends, and now only the top of buildings and pixel arts remain.
Luckily, I have a backup of it that I have not loaded in this update. Hopefully they fix this soon so I can play on it again.

I was working on a modded minigame and the entire thing destroyed itself because of this glitch.
everything was gone

Same thing happened to my world, I hope there is a fix for this in the near future.

New update is now out, the bug if fixed. Even if you didn’t make a backup your world will be fixed

The holy Jerusalem was destroyed because of this. We will take it back...
and not use experimental features on it again.