Hi.

1. Item flag "item-modifiable" doesn't work if player quickly took off his armor and put it an the anvil. After which he can modify item. I would like to suggest instead of banning the placement of this in the anvil slot cancel forging event.
2. I would like to suggest adding in this item flag (or separately) prohibition of use grindstone and command action "item is broken"

Server Version: Paper-1.19.3

I confirmed the issue and it is now fixed in the latest release, in addition, I added the grindstone as requested.

Link; https://www.spigotmc.org/resources/itemjoin.12661/download?version=494934
Remember to remove your old ItemJoin.jar file

If you continue to experience issues feel free to open a new issue and include the /itemjoin dump information.