When using the "replaceitem" command to set an item to the off hand slot (slot.weapon.offhand), When you use the key "F" to switch the items, it deosnt work.
How to reproduce:
1.type in the chat: /replaceitem entity @p slot.weapon.offhand minecraft:grass
2. press the key F or what ever else is set to the "swap items in hands" key.
3.LOOK AT THE SCREEN