When holding a casting item in main hand and a useable item in off hand, right clicking will both cast a spell and use the off hand item. I believe this is because the casting item returns PASS instead of SUCCESS on client side, which allows the interaction to be handled by the off hand. Any item that does client-side handling on use, such as the staves from Iron's Spells n Spellbooks, will have at least partial activation. This is also likely related to other inconsistencies when right clicking a casting item on an interactable block or entity.

Related issues:
#992
#887

Got home and tested it, changing the return enum changed nothing. I'll keep investigating.