How firely sdk handle extra fields (not extension) in Poco and ISourceNode

Most time, I think json is forward and backward compatible as long as some convention is kept such as adding fields only etc. I always thought so since I studied CQRS and event sourcing. Is it true for FHIR, especially firely sdk? I answered so in a discussion, however I doubted it as I didn’t do it in FHIR. Here I have a short code to demostrate the two case. In the end, the result is yes and no. For poco case, firely sdk will raise an exception; on the contrast, ISourceNode can parse the data successfully. It will fail when calling ToPoco from ISourceNode as well. Now there is one open question how do we persist data ? via POCO or ISourceCode? Do we allow those extra fields?