Hi Jennifer,
You are saying the external reference field is empty in your data & analysis of completed surveys, correct? I would check and confirm that your responses were successfully authenticated/tied to the contacts in the mailing list. If this survey was sent out via email, is the email field populated in the survey response? If there were embedded data associated with the contact list, are those fields populating in the survey responses?
If it is just the external reference missing, I do find that odd. But if there is no data associated with the contact, it is possible that they did not use a personalized link to access the survey.
The external data reference field is blank in the distribution history.