If you /item replace your offhand item with air this item will forever remain in limbo of being shown by /data get entity @s Inventory, but being unremovable as you can't modify player data.

/clear, or replacing the item back to take it out normally also doesn't work. /clear only see items that are in your inventoy not those limbo items and /item replace or just placing an item in your offhand create another instance of Slot:-106b in your Inventoy data.