This bug is about duplicating items through game crashes.

The game crashes or forcibly stops the game within a few seconds after the player places an item in a crate or drops an item, or dies causing the item to drop, and all of the player's states will revert back to before this all happened after re-entering the game, but the items that the player put in the crate or dropped will be retained, thus achieving the effect of item duplication.

I think the general idea of how this feature works is that I happened to save /region when the game auto-saved the progress, but the /playerdata didn't, and the game crashed due to a memory overflow, resulting in items in chests and furnaces being in the last minute state before the game crashed, and the player information was still in the same state as the last time the game was autosaved, thus duplicating the items.

This feature works with third-party launchers that support quick shutdown of the game, but can sometimes cause items to be lost.