I don't deal with external APIs often, but this is a development nightmare. You can't just magically let data flow through your system without knowing about it, because this is not how programming works. Your API has a contract and your code is written to support that contract, if the contract changes it should either be a very consciously decided breaking change that is versioned somehow, or it should be an unversioned non breaking change. Apparently whatever data is added like this is completely meaningless to your program so why do you need to be in charge of passing it back to the API.

Changing your API and assuming everything just keeps working is a nonsense cowboy attitude to software compatibility, even if some frameworks bend over backwards to support it through magic that's hidden from the developer. Furthermore, many programming languages are simply incapable of doing this, and this approach to APIs is immediately restricting those languages from use.

Finally, transforming objects to an internal domain model is really the cornerstone of a lot of recent well-thought-out programming discipline, and this API design is throwing that in the garbage. It's explicitly asking you to mess up your service architecture, spreading bad architecture like a virus to all systems that interact with the API.