Whilst you cannot mine- with offhand- it will break leaves, crops and flowers.
I would urge you strongly that you make it so offhand cannot do this OR make it so that we can disable it via console.

As breaking these blocks via offhand- will not be considered by the server as the player themselves actually breaking the block.
This prevents plugins / mods intended to prevent grief from being able to cancel the block break event for specific players.

Ideally, breaking anything with the offhand should behave like you've broken something with your main hand.