A form has many fields. Association are include in data.fields instead of data.included.

model-field.js

import DS from 'ember-data';
import { computed } from '@ember-decorators/object';
import { attr, belongsTo } from '@ember-decorators/data';

export default class Field extends DS.Model {
  @belongsTo('form', { async: false }) form

  @attr controlType
  @attr label
  @attr placeholder
  @attr values
}

model-form.js

import DS from 'ember-data';
import { hasMany, attr } from '@ember-decorators/data';

export default class Form extends DS.Model {
  @hasMany('fields', { async: false }) fields
  @attr() updatedAt
}

result

{
  "data":{
    "attributes":{
      "updated-at":"2018-10-20T23:08:28.657Z"
    },
    "fields":[
      {"data":{"id":"1","attributes":{"control-type":"text","label":"1","placeholder":"1","values":"1"},"relationships":{"form":{"data":{"type":"forms","id":"1"}}},"type":"fields"}}
    ],
    "type":"forms"
  }
}

serializer-form.js

import DS from 'ember-data';

export default DS.JSONAPISerializer.extend(DS.EmbeddedRecordsMixin , {
  attrs: {
    fields: { embedded: 'always' }
  }
});