Use the Command on the Inventory of Entity.
(Minecart Chest, Donkey, mule...)

Need to Close and open the GUI.
Also after a "/data modify" the command /replaceitem dosen't work.

https://youtu.be/NX1VgpjQYXY

Exemple :

/data modify entity <Entity> Items[].tag.Enchantments[] set value {id:"infinity"}

On the Block (Chest, ShulkerBox...) use /data modify work fine.
Just Entity doesn't work.

Really problémactic for Custom UI.