If you give an attribute to the off hand, there is a bug where the off hand attribute is not applied when using an item with the same attribute in the main hand.
Prepare two items with different names and the same ability.
One on the left and one on the right.
Afterwards, empty your main hand.
The off hand ability disappears.

Will fix it soon by adding a custom uuid option!

While testing I found the exact same issue, here are the steps I took to reproduce it:

• Get 2 instances of the same item, a stat increase while held in the offhand.
• Have one in your offhand, and the other in your mainhand. This causes the bonus to not work.
• Switch to a different item, or nothing in your mainhand, the bonus still doesn't work.

There was also another issue where items that had modifiers while in your offhand would not show "When in Offhand:" before the offhand stat modifiers.

my best guess for the former issue is that there is a issue with how the mod handles multiple types of the same item.

Fixed on 2.0.0, please read full changelog as the config moved locations and the format also changed.
Custom UUID is not needed