If you do not want the new question/field added to your existing feature service, you need to set the bind::esri:fieldType for that hidden question to "null". See my screenshot and example xlsx file. When null is specified in that column, the feature layer does not get updated when you published, and when you submit records, so data is attepted to be submitted for that question/field. From your screenshot above, it appears you did not set the "null".