normalizeResponse not recognizing link between nested relationships

The API endpoint I'm working with is returning data that has multiple nested relationships inside it, and I am using normalizeResponse() within DS.JSONAPISerializer to massage it into something that is fully JSON-API compliant.

The ember inspector shows that all data gets placed within its respective container correctly. The link between the top-level model and its hasMany child does work, but the link between the nested models does not work. I verify this by navigating within the inspector to the nested model's child model, clicking on it, and observing that its 'content' property is null.

First, take a look at how my models are set up:

// models/search.js
// i am able to browse from the search model to children with success
export default DS.Model.extend({
  articles: DS.hasMany('article'),
});

// models/article.js
// i CANNOT browse from an article down to its digest in ember inspector
export default DS.Model.extend({
  search: DS.belongsTo('search'),
  type: DS.attr(),
  created: DS.attr(),
  updated: DS.attr(),
  digest: DS.belongsTo('digest'),
});

// models/digest.js
export default DS.Model.extend({
  title: DS.attr(),
  desc: DS.attr(),
  date: DS.attr(),
  article: DS.belongsTo('article'),
});

Now, here's my modified JSON after my functions inside normalizeResponse complete. AFTER returning this data from normalizeResponse, the "digest" object under the parent "relationships" object disappears. Is there something wrong with my JSON? I've tried so many permutations of this with no success, and I am pretty sure this matches the JSON-API spec for Compound Documents.

{"data":{
  "type":"searches",
  "id":"17482738723",
  "attributes":{

  },
  "relationships":{
    "articles":{
      "data":[
        {
          "type":"articles",
          "id":"19988"
        },
        {
          "type":"articles",
          "id":"19989"
        },
      ]
    },
    "digest":{
      "data":[
        {
          "type":"digest",
          "id":"19988_digest"
        },
        {
          "type":"digest",
          "id":"19989_digest"
        },
      ]
    }
  }
},
"included":[
  {
    "id":"19988",
    "type":"articles",
    "attributes":{
      "type": "internal",
      "created":"2016-09-27T00:13:11.000Z",
      "updated":"2016-09-27T00:13:11.000Z",
    }
  },
  {
    "id":"19988_digest",
    "type":"digest",
    "attributes":{
      "title":null,
      "desc":"four five six",
    }
  },
  {
    "id":"19989",
    "type":"articles",
    "attributes":{
      "type": "internal",
      "created":"2016-09-27T00:13:11.000Z",
      "updated":"2016-09-27T00:13:11.000Z",
    }
  },
  {
    "id":"19989_digest",
    "type":"digest",
    "attributes":{
      "title":"one two three",
      "desc":null,
    }
  },
]
}