Ember 3.25 BelongsTo not sideloading

I am trying to sideload relationship data from one API call. I have a my two models setup like so:

Contest Model (child, belongsTo):

import Model, { attr, belongsTo } from '@ember-data/model';

export default class ContestModel extends Model {
  @attr('date') createdAt;
  @attr('string') description;

  @belongsTo('brand', { inverse: 'contests' }) brand;
}

Brand Model (parent, hasMany):

import Model, { attr, hasMany } from '@ember-data/model';

export default class BrandModel extends Model {
  @attr('date') createdAt;
  @attr('string') name;

  @hasMany('contest') contests;
}

I'm fetching via this line:

this.store.findRecord('contest', params.contest_id, { include: 'brand' });

This is the returned API payload:

{
  "data": {
    "type": "contest",
    "id": 1,
    "attributes": {
      "body_json": [
        {
          "question": "what time is it?",
          "answers": ["1PM", "2PM", "3PM", "4PM"]
        },
        {
          "question": "which collection is this artwork from?",
          "answers": ["botanical garden collection", "ghibli collection", "adventure collection", "swag collection"]
        }
      ],
      "created_at": "2021-05-23 18:00:00 -0700",
      "description": "a good description goes here"
    },
    "relationships": {
      "brand": {
        "links": {
          "self": "http://example.com/contests/2/relationships/brand" // not sure what this does
        },
        "data": { "type": "brand", "id": 2 }
      }
    },
    "links": {
      "self": "http://example.com/contests/2" // not sure how this is useful
    },
    "included": [
      {
        "id": 2,
        "type": "brand",
        "attributes": {
          "created_at": "2021-05-23 20:00:00 -0700",
          "name": "aloha vibe"
        }
      }
    ]
  }
}

The issue here is that calling myContest.brand returns a Proxy object just like in this SO post.

Is my API payload incorrect or is something misconfigured for my models? Or something else? I'm on Ember 3.25.3