{original} refers to the original file name that was stored to xattr metadata the first time the file was renamed with FileBot. {original} is always undefined if the file at hand has never been renamed with FileBot.
{original} did indeed default to the current filename, but that made the binding less versatile and less well-defined and prone to misinterpretation. It can be a problem when a bindings work one way in one context, and another way in another context.
You can use {original} and explicitly default to {fn} if undefined like so: