1. When you run command with a disposable item on your "offhand", the item will not be consumed.

• right-click
• multi-click
• inventory

2. Left-click item on your "offhand" will not get anything.

3. If item has no name, it will casue error.
   https://pastebin.com/PfD3m1cY

4. There is one more ' in the item.yml, it will not stop and display lots of error in console until you fixed it.
   https://pastebin.com/upLBtuYD

This is the example item list.
https://pastebin.com/J7Yz6Cn8

ItemJoin version: 4.9 (build #147 latest)
Spigot Version: git-Spigot-642f6d2-fbe3046 (MC: 1.12.2) (Implementing API version 1.12.2-R0.1-SNAPSHOT)

I have tested this build and it seems to be working properly now; http://ci.craftationgaming.com/job/ItemJoin/155/

Also one of the items you listed as a "no-name" saying it causes an error I extensively tested with no errors appearing. If this error occurs again please send me the error code in a pastebin link so I can resolve the issue.

Let me know if this has resolved your issues and I apologize for the late update. It has been hard to find time this past quarter between college and the new ItemJoin v3 rewrite to be able to respond quickly.

The latest version I tested showed no problem.
Thank you very much.

Besides, I found a little issue.
When you use the custom items on your hand and off-hand at the same time, only the command of the item on hand will run.
If the off-hand item is disposable, it will be consumed and not run command when the server get a little lag.

ItemJoin version: 4.9 (build #155 latest)
Spigot Version: git-Spigot-642f6d2-fbe3046 (MC: 1.12.2) (Implementing API version 1.12.2-R0.1-SNAPSHOT)

Example items
https://pastebin.com/tE5dT7ju

Testing video
https://youtu.be/ipkPAWCJFjU

I apologize for the late reply. It has been hard for me to jump back and forth between the new and old framework since ItemJoin v3 (v5.0.0) is close to exiting development. Each time I jump between frameworks I have to re-learn their systems and structures so I can properly debug it.

So I am a little confused on what the issue is exactly.
1.) When you use the custom items on your hand and off-hand at the same time, only the command of the item on hand will run.

If this is an issue you are referring to this is intended so the main hand runs before the offhand.
(are you wanting both to run?)

2.) If the off-hand item is disposable, it will be consumed and not run command when the server get a little lag.

I wasn't ever able to replicate this. Did you double check that it is not a visual glitch by clicking your cursor in the (off-hand) item slot and seeing if it re-appears. Also, re-logging would show if its a visual issue.

EDIT: Finally was able to replicate this. Turns out its not a lag issue. It happens at certain angles of the command execution. If you have both items in your hands (spamming the commands) and at a normal speed drag your cursor from facing directly at your feet to far in front of you (about half-way) you will notice the item disappear. This is definitely not a visual glitch and is a bug.

Issues should be resolved for you in this snapshot; http://ci.craftationgaming.com/job/ItemJoin/157/

This issue has been officially fixed in the release of ItemJoin v5.0.0.
You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=254090

Please read the changelog as the documentation has not been updated yet;
https://www.spigotmc.org/wiki/recent-changes

New Example items.
https://pastebin.com/3Qbk0vwN

Testing video.
https://youtu.be/iilmpVqSf_4

Thank you for the video, I was struggling a bit to figure out exactly what the problem was.
I think I figured out a fix for the issue. I will post back as soon as I have a snapshot available in the next few days.

Also, I apologize for the delay, it has been a really busy Fall Quarter at college.