In my case I did everything you suggested. I have successfully managed to add a new currency through > Studio > Forms > Data Items > Payment > Currencies. and i can see in the new currency there.
Also the new currency shows in the Payment App.
But when I go to > Studio > Seetings General > Currency code (for checkout system) > There are only two currencies displayed: USD and EURO. I don't see the new Currency I added in Form (Data Items). Did I miss something?
Why the new currency doesnt show there (Seetings > General > Currency code (for checkout system) and Where to change the site default currency? Thanks