Item(command) bug

use command "item"
"item entity @a weapon.offhand copy entity @a[limit=1] weapon.mainhand"
Open the Inventory and take out any item to put it in the mainhand. Then the offhand will display the item of the mainhand. But open the Inventory again, switch to the tab outside the Survival Inventory. Take the mainhand item, and the offhand will still show the original item. Close the Inventory, the items of the offhand cannot be used. Just like a texture