We have been resolving this issue with Pedro via email. I am posting the solution in case anyone else comes across the same scenario.
This property is not accessible like the SKU object, because it is not registered as an object in the web part data source. This means that only its string representation is available for Eval. The reason why the SKU object is accessible is because it is Object dependency of ShoppingCartItemInfo object, so it is loaded automatically.
As a workaround, you can use this statement in your transformation:
This will find the SKUInfo object among the Site SKU's based on the Parent SKUID.