Totally makes sense. However, I don't remember doing that so I don't know how to fix it. Where are those options hiding? I'm assuming somewhere in the databases.
Viewing post in Combining Item Plugin for RPG Maker MZ comments
Just delete the plugin and download it again. This should work. One unpleasant bug has also been fixed in the new version.
If the problem remains, then go to the "Plugin setting", double-click on "action_name" and make it so that there are three commands. The first is responsible for using the item, the second for combining, and the third for the back. You can call it whatever you want, the main thing is the order of the commands.