It seems like there is a regression from version Ember Data 3.27.1 to 3.28 when requesting a model simultaneously.
I tried to create a repo which could demonstrate the problem. Be aware that this is only assumptions and that I don't know if it's really a bug or a user error or whatever. So please take everything I write here with a grain of salt (because it could be totally wrong as well).
From my point of view it boils down to the following code:
// taken from the file: app/routes/catalog.js
const catalogPromise = this.store.findRecord('catalog', 'usm');
// This "later" just mimics what happens in our app somewhere else.
// It's a huge app and the catalog is a central model so we
// need it in several places, therefore it can happen that
// there are two simultaneous calls to try to find a specific
// catalog
later(async () => {
const catalog = await this.store.findRecord('catalog', 'usm');
console.log(catalog);
}, 1500);Since our app is grown over 5 years and the catalog model is a very central model it could happen that we request it on several places at the same time. It seems like if the calls are simultaneous the method updateRelationshipOperation is called for each simultaneous call and the second call to updateRelationshipOperation results in false for relatedLink && relatedLink.href && relatedLink.href !== currentLinkHref so hasUpdatedLink becomes false and furthermore the isStale property is set to false. This leads to the problem that the relationships are not loaded if you actually request them.
I created a screencast to demonstrate the issue.
You can find the screencast for 3.28.3 here
the screencast for 3.27.1 is here