When I have any item in the off-hand slot, and I right click in air, the off-hand event reports to use the EquipmentSlot.HAND (instead of OFF_HAND).

Example: Wool block in off-hand, Stone in main hand, right clicking air I get those two events:
[18:20:30 INFO]: PlayerInteractEvent|RIGHT_CLICK_AIR|ItemStack

{STONE x 1}

|mainhand:true
[18:20:30 INFO]: PlayerInteractEvent|RIGHT_CLICK_AIR|ItemStack

{WOOL x 1}|mainhand:true

Another example: Wool block in off-hand, nothing in main hand, right clicking air I get this event:
[18:21:54 INFO]: PlayerInteractEvent|RIGHT_CLICK_AIR|ItemStack{WOOL x 1}

|mainhand:true

The events with the wool block in it should report 'mainhand:false' instead.

For right clicking blocks I get the correct EquipmentSlots though.