The Smart Editor widget in ArcGIS Online shows all the attributes of a feature service including the hidden attributes in the widget settings, when the Edit settings feature service is set to the 'Add Features only' option. This disallow users to hide information that are not for public users when adding data through a web app.
Change the Edit settings feature service where the options are either 'add, update, and delete features' or just 'add and update features.' Selecting one of these options respects the hidden field and keep the information hidden from public users when the users are looking at the features in a map. However, users are able to edit other fields that are still exposed.