With the Offhand, I've noticed two bugs related to it.
The first is that if a Ranged Weapon is set use itself as ammo, and the weapon/ammo is placed only in the offhand, it won't shoot because it is not checking the offhand for ammo in survival. Relevant element is named Corn.
The second is if I use a procedure that removes an item from the offhand, it will first remove any of the same item from the main inventory and remove it from the offhand last. Relevant element procedure is named MonarchButterflyRelease, and the item it uses is called the Captured Monarch Butterfly. Moved to a separate issue page.
Most likely the offhand isn't being checked properly.
Issue comments
The second is if I use a procedure that removes an item from the offhand, it will first remove any of the same item from the main inventory and remove it from the offhand last. Relevant element procedure is named MonarchButterflyRelease, and the item it uses is called the Captured Monarch Butterfly.
This is a separate issue so open a separate ticket. The procedure is huge so make an isolated workspace with minimal needed elements (likely one procedure) that demonstrates this problem isolated.
Your workspace uses plugins which makes it hard to debug. Please avoid this next time when reporting bugs. Make a minimal example of the workspace that is vanilla MCreator.
I will test this when I get time.