Description
When a world created in 1.21.11 or an earlier version is upgraded to 26.1 or a later version, the Overworld of the upgraded world is populated with already-generated chunks, player data, and advancement data that belong to a completely different save file instead of the original world’s own data. The foreign data persists even after all other save files are deleted from the saves folder, which makes the issue very persistent and suggests a problem either in the new world format migration or in the datafixer/upgrade code (possibly misidentifying or caching the source world during the 26.1 world format restructure).
Steps to Reproduce
Have at least two worlds in the saves folder: World A (created in 1.21.11 or earlier) and World B (any world).
Launch the game with version 26.1 or later (e.g., 26.2).
Open World A and allow the world upgrade to complete.
Enter the Overworld of World A and inspect the terrain, player inventory/position, and advancements.
(Optional) Delete World B from the saves folder entirely, restart the game, and open World A again.
Observed Results
After the upgrade, the Overworld of World A contains chunks, player data, and advancement data that belong to World B (or another save file) rather than World A’s own data. The incorrect data still appears even after all other save files have been deleted from the saves folder, and the issue cannot be resolved by normal means.
Expected Results
After upgrading World A to 26.1 or later, the Overworld chunks, player data, and advancements should be migrated from World A’s own pre-upgrade data, and the upgraded world should remain fully independent of any other save files in the saves folder.
Attachments
Comments 10
Thank you for uploading a video! Could you please also attach the affected world? If it is too big, you could upload it to OneDrive and link it here.
Thank you for attaching the world!
Did you by any chance open the world in an older version after saving it in a newer version?
This bug seems a very serious (in fact, a critical) one, however I haven't noticed such a strange damage (I made some test worlds).
Does this bug happen in a regular way, for most or even all upgraded worlds ?
Or, maybe it just happened as a unique/exceptionally, for the specific world that was attached in the bug report ?
It looks a weird to me that such a bug was reported only on 20/Aug/2026, when the 26.1/26.2 updates have already been used for some months, and probably many players have already upgraded their worlds and how they did not notice such damages ??
This issue would not happen by itself, but only when manually copying/extracting worlds and using the “replace all” functionality of the file browser. This does not replace entire folders, but merge the files inside them, only replacing those that exist in both.
The game then could get confused about the mix of different world files inside the folders.
It’s not easy to entirely resolve the root of this issue, as we also try to preserve as many files in the world folder as possible when upgrading.
So the fix is just an improvement to how the case of finding a weird mix of files is handled and communicated.
Thank you for your report!
However, this issue has been temporarily closed as Awaiting Response.
Could you please attach the world in which this issue can be observed?
This ticket will automatically reopen when you reply.
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