Yes i know ,thank you.
I actually found it in the debug while trying to figure out what's happening
img
And , it's a bit bizzare. Because further while NBT try's to save the list of the items, it just writes minecraft:air ignoring that the itemstack item is diffirent.
I tryes the default implementation and a custom with just .writeNBT from the itemstack methods, and it's the same, it writes minecraft:air all the time no matter the item.