I played on a server where the duping worked perfectly fine. However recently (1.19.4), I do the same thing but when I click on Close without Packet, it closes with packet. It drops the item on the ground like normal but when I open the Ender-Chest again, the item isn't there which means it isn't duping anymore.

that's just called they fixed the dupe, nothing is wrong with the mod lol.

ask the server owner to add a dupe plugin if you want, because I think they patched it for a reason.