1. create an custom anvil inventory like this:
Bukkit.createInventory(null,InventoryType.ANVIL,"Enter name");

2. Add an item to the first slot
3. let a player open it
4. start a scheduler (in a clickevent/openevent) monitoring all anvil slots
5. move around the items in the anvil
-> the items doesnt update in the server side inventory

Redo this with an actual anvil thats standing in the world and the inventory will update fine on the server side