EmberJS Data Assert in setRecordDataFor "Illegal set of identifier"

Using 3.28 I am hitting an assert deep inside Ember Data when retrieving a relationship on one of my models. I can run the following query just fine:

return this.store.findRecord('project', project_id)

However when I run a subsequent

const monitors = await model.monitors;

in my afterModel() of the route, this assert is triggered.

My Models look pretty standard:

export default class ProjectModel extends Model {
  @attr('string') name;
  @belongsTo('user') creator;
  @attr('date') created;
  @attr('date') changed;
  @hasMany('domain-monitor') monitors;
}

export default class DomainMonitorModel extends Model {
  @belongsTo('project') project;
  @hasMany('page-monitor') pages;
  @attr('string') protocol;
  @attr('string') domain;
  @attr('date') created;
  @attr('date') changed;
}

If someone can point to what is going wrong, I am happy to submit an MR to improve this assert message to hopefully unstuck someone in the future. Thanks!