So I also tried it with the sample Python implementation of the National Rail community and it did also not work. Hence I am 100% the error lies in the Zeep package parsing.

Workaround: I used the Zeep HistoryPlugin in order to receive the raw XML from the request. Then i manually parsed the type, using lxml package, into my Zeep reponse. This works pretty good if you only need one or two additional types.