https://github.com/Alex-the-666/Rats/blob/1.16.1/src/main/java/com/github/alexthe666/rats/server/events/CommonEvents.java#L298

Here, worlds are inserted into the global map but never cleared from it, meaning every save you open in single player never gets cleared from memory until you restart the game.

Recommend a change to a WeakHashMap which will automatically expire entries as worlds get garbage collected