the data get detects items not present (créatif)

step_1 : In creative

step_2 :take any item and then go to "inventory" and
step_3 :put your item in either the armor slots or in the second hand.
step_4 :Then do a "/data get entity @s Inventory"

You will have 2 items instead of 1 (because if "/clear @s", the 2 items will be clear) else

step_5 : if deco-reco (quit world and rejoin), second item reappears in hotbar