I wanted to make a datapack that replaces an item with another one when getting an advancement. So I made an advancedment that runs a function as a reward wich clears a specific item, then gives the player an other item and then removes the advancement from the player .The function itself runs perfectly if executed by the player but if executed by the advancement the function creates a ghost item. The ghost item gets detected by the advancement. If you move the ghost item around in your inventory you get the advancement over and over again (because the function removes the advancement again). The ghost item only disappears if you open a chest, furnace etc.. I made an example datapack wich converts stone into diamond blocks. Just grab a stone out of the creative inventory and you will see the problem.