Enter the following command to give yourself an item with a custom minecraft:item_name component:

/give @s minecraft:diamond[minecraft:item_name='"item name"']

Then drop the item and enter:

/say @e[type=minecraft:item,limit=1,sort=nearest]

It prints "Diamond" to the chat instead of "item name".

(The same applies to the minecraft:custom_name component, but I suspect that's not a bug since that's never worked before.)