This can happen if the security category of the hidden interest update is not set to "general". When you go to edit the survey:
  1. Go to step 2, "Include Questions"
  2. Next to the hidden interest update question click "Edit"
  3. For step 4, "Security Category", choose "General"
Next to the security category it says "Controls who the question displays to. The question updates the interests for all users who take the survey, even if the question is not set to display to them." but this message is misleading. A Hidden Interest Update question will never display to a constituent, even if they are in that security category. You can still set the security category for the interest itself to be restrictive so that constituents will not see that they are subscribed to it, but the survey question should be set to "general".