Updated relationship data not sent to server with Ember and Ember Data

I took over an app written in Ember, which is fairly new to me. Also, I learned Ember Octane, but the app is using Ember 3.2. It also uses Ember Data with the default JSONAPISerializer.

In my app there's a project model, and each model belongs to a creator, which is an instance of a person:

In models/projects.js:

creator: DS.belongsTo('person', { inverse: null }),

I want to update this relationship data (assign a new creator to the project) in a controller method:

project.set('creator', otherPerson); // or project.creator_id = otherPerson.id;
project.save();

The POST request is successful, but there isn't any relationship data in the payload, hence the creator isn't updated.

What's wrong here?