Direct answer

If only advancement progress resets, inspect the affected world’s advancements/<player-uuid>.json files and server save errors. Check whether inventory and position persist before assuming all player data is lost.

What it means

Java Edition stores advancement progress separately from the main player inventory NBT data. A world-folder switch, changed UUID, read-only storage or modded advancement replacement can make previously earned advancements disappear.

If you are a player

  • Complete one low-risk advancement and reconnect once to see whether it persists.
  • Report the exact username, time, advancement and whether inventory or position also rolled back.

If you run the server

  1. Stop the server and preserve a backup before modifying advancement JSON.
  2. Confirm the active world path and the matching account UUID; avoid guessing from player display names.
  3. Inspect the advancement JSON timestamp and server logs for file-write or JSON parsing errors.
  4. If the entire world rolled back, investigate shutdowns and disk errors; if only advancements did, isolate datapacks or plugins affecting advancements in a test copy.

Confirm the fix: After a clean shutdown and restart, obtain a small advancement and confirm it is still completed. Restore files from a consistent backup rather than hand-editing a production JSON file.

References

Use the documentation for the exact software and release you run. These sources support the settings or diagnostic behavior used in this fix.

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