Migrating world creates 00000000-0000-0000-0000-000000000000.dat player file

Expected behavior

No extraneous playerdata file created.

Observed/Actual behavior

After migrating a world from 1.21.11 to 26.1.1, there is a file named 00000000-0000-0000-0000-000000000000.dat in plotworld/players/data. This data is also returned by calls to getOfflinePlayers() as if it was an actual player, however the playername is null.

After deleting the file and restarting the server, the extraneous file does not return, so it seems to be only during the migration process.

Steps/models to reproduce

I have a single world named plotworld. No Nether, no End. Create a server like that with 1.21.11 files and let it migrate, then look at the plotworld/players/data folder.

Plugin and Datapack List

Bukkit plugins (29):

• AltDetector (2.04), BetterSeen (1.03), BlueMap (5.18), BlueMapFilteredEntities (1.0), BookExploitFix (1.4), Essentials (2.22.0-dev+75-8e3b24f), EssentialsChat (2.22.0-dev+75-8e3b24f), EssentialsGeoIP (2.22.0-dev+75-8e3b24f), EssentialsSpawn (2.22.0-dev+75-8e3b24f), FastAsyncWorldEdit (2.15.0+59372c5), ImageMaps (1.0.10), IronDoors (1.8c), LimitDrops (1.05), LuckPerms (5.5.42), MemCheck (1.04-SNAPSHOT), NBTAPI (2.15.6), PlaytimeScoreboard (0.03), PlotMarkers (2.00), PlotSquared (7.5.12), PlotWelcome (0.03), PwnFilter (3.1.2c), TabListPing (2.03), Vault (1.7.3-b131), ViaBackwards (5.9.0-SNAPSHOT), ViaVersion (5.9.0-SNAPSHOT), ViaVersionStatus (4.00), WallClock (0.2.2g), WorldGuard (7.0.16+2355-f7fded2), prism (4.3-dev2)

Paper version

Paper 26.1.1-16-dev/26.1@f9da803

Other

My test server is running Windows 11. Here's part of the startup log:

2026-04-04 19:37:56 [INFO] Starting upgrade for world "plotworld"
2026-04-04 19:37:59 [INFO] Upgrading progress: 2%
2026-04-04 19:38:00 [INFO] File system capabilities: FileSystemCapabilities[atomicMove=true, hardLinks=true]
2026-04-04 19:38:00 [INFO] Applying file structure changes for world "plotworld"
2026-04-04 19:38:00 [INFO] Moving new hardlinked world to top level
2026-04-04 19:38:00 [INFO] Start cleanup
2026-04-04 19:38:00 [INFO] Moving out old world folder
2026-04-04 19:38:00 [INFO] Moving in new world folder
2026-04-04 19:38:00 [INFO] Done applying file structure changes for world "plotworld". Cleaning up outdated data...
2026-04-04 19:38:01 [INFO] Upgrade done for world "plotworld"
2026-04-04 19:38:01 [WARN] [WorldFolderMigration] ===================== ! ALERT ! =====================
2026-04-04 19:38:01 [WARN] [WorldFolderMigration] World storage migration is required during startup.
2026-04-04 19:38:01 [WARN] [WorldFolderMigration] If you do not have a backup: interrupt the server now. Use Ctrl+C, your panel kill function, etc.
2026-04-04 19:38:01 [WARN] [WorldFolderMigration] =====================================================
2026-04-04 19:38:01 [WARN] [WorldFolderMigration] Continuing in 30 seconds...
2026-04-04 19:38:31 [INFO] [WorldFolderMigration] Continuing with startup world migration.
2026-04-04 19:38:31 [INFO] [VanillaWorldMigration] Starting Vanilla import for world 'plotworld' (minecraft:overworld)
2026-04-04 19:38:31 [INFO] [WorldMigrationSupport] Migrating Paper world config from .\plotworld\paper-world.yml to .\plotworld\dimensions\minecraft\overworld\paper-world.yml
2026-04-04 19:38:31 [INFO] [VanillaWorldMigration] Completed Vanilla import for world 'plotworld' (minecraft:overworld)

Reproduce the migration using the stated 1.21.11 and 26.1.1 world setup, then start at the VanillaWorldMigration entry point shown in the startup log. Inspect the migration around the world’s players/data directory and verify that no zero UUID file is created and getOfflinePlayers() does not return a null-named player.