There are a few attributes on the Idea object which have an API Attribute ID, however they are not appearing in the response for the API call (example https://your_server/ppm/rest/v1/ideas/5056068)
We hypothesized that perhaps this is the case because they have no data, but other attributes are returned in the API response with null if they have no data.
Release : 16.0.2
Component : Clarity XOG and REST API
Attributes were secured with no group given access to them. Once Admin group was given view rights attributes were returned with API request.