Direct answer

The official Leaf 1.21.1 reference explicitly warns that async-playerdata-save can cause data loss in some circumstances. Faster writes are not worth risking inventories, locations and progression on a production server without a tested recovery path.

How to check it

  1. Look at save-related delays and storage metrics before blaming player-data writes; GC, plugins and slow disks can cause similar symptoms.
  2. Confirm the exact Leaf release and presence of the generated async.async-playerdata-save key.
  3. Take a complete offline backup and perform login, logout, restart and restore tests using a disposable copy of player data.

What to change

For live servers, prefer leaving the experimental feature disabled unless your exact build documents its safety and your tests justify the risk:

async:
  async-playerdata-save:
    enabled: false

Address filesystem permissions, disk latency and backups first; never switch it on simply because other async options improved performance.

Compatibility and safety

The explicit data-loss warning comes from the Leaf 1.21.1 documentation. More recent Leaf versions may change or remove the setting; verify the exact build rather than assuming an old config applies.

Software and version notes

The cited Leaf global config page explicitly describes the 1.21.1 branch; newer Leaf builds may have different keys, defaults or feature support. Verify config/leaf-global.yml generated by your actual server.

Verify the result

Compare saved inventory, ender chest, location and player statistics after clean and unexpected restart tests. If any mismatch occurs, stop testing and restore the verified backup.

References