ED 3.1.1 bug repro

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

controllers.index.js

import Ember from 'ember';
import { inject as service } from '@ember/service';

export default Ember.Controller.extend({
  store: service(),
  actions: {
    addItem(event) {
      event.preventDefault();
      let item = this.store.createRecord('item', {order: this.order});
      item.save().then((itm) => {
        console.log('saved the item!', itm);
      });
    }
  },
});

mirage.config.js

let id = 0;

export default function() {
  window.server = this;

  this.post('/orders', function (schema, request) {
    id = id + 1;
    const resp = {
      "data": {
        "attributes": {id},
        "id": id.toString(),
        "type": "order"
      }
    }
    console.log('creating order id: ', id);

    return resp;
  });
  this.post('/items', function (schema, request) {
    id = id + 1;
    const resp = {
      "data": {
        "attributes": {id},
        "relationships":{"order":{"data":{"type":"orders","id":"1"}}},
        "id": id.toString(),
        "type": "item"
      }
    }
    console.log('creating item id: ', id);
    console.log(request);
    return resp;
  });

};

models.item.js

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

export default Model.extend({
  order: belongsTo('order', {inverse: 'items'}),
});

models.order.js

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

export default Model.extend({
  items: hasMany('item', {inverse: 'order'}),
});

routes.index.js

import Ember from 'ember';

export default Ember.Route.extend({
  setupController(controller, model) {
    this._super(controller, model);
    let order = this.store.createRecord('order');
    order.save();
    controller.set('order', order);
  },
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

templates.index.hbs

Yo

<a href onclick={{action 'addItem'}}>
Click here to add an item
</a>
<h1>Order id: {{order.id}}</h1>
<ol>
{{#each order.items as |item|}}
 <li>item id: {{item.id}}; item internal model id: {{item._internalModel.id}}</li>
{{/each}}
</ol>

twiddle.json

{
  "version": "0.15.0",
  "ENV": {
    "ember-cli-mirage": {
      "enabled": true
    }
  },
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.1.2",
    "ember-template-compiler": "3.1.2",
    "ember-testing": "3.1.2"
  },
  "addons": {
    "ember-data": "3.1.1",
    "ember-cli-mirage": "0.2.4"
  }
}