ternary relationship with @ElementCollection - left join fetch misbehaving

Question

JPA 2, Eclipselink 2.3.2

I'm trying to model a ternary relationship with two @ElementCollection as opposed to two @OneToMany's mainly so I can make the junction object an @Embeddable instead of an @Entity and avoid needing an @Id for the nested object.

For example

@Entity
public class Project {
   @ElementCollection
   @CollectionTable(name="assignment", joinColumns=@JoinColumn(name="project_id"))
   public Set<Assignment> assignments;
}

@Entity
public class Employee {
   @ElementCollection
   @CollectionTable(name="assignment", joinColumns=@JoinColumn(name="employee_id"))
   public Set<Assignment> assignments;
}

@Embeddable
public class Assignment {
  @ManyToOne
  @JoinColumn(name="employee_id")
  public Employee employee;

  @ManyToOne
  @JoinColumn(name="project_id", insertable=false, updatable=false)
  public Project project;
}

To persist, I am adding Assignment instances to Project instances, with reference to the right Employee. This works fine.

Further, I can reference the list of Projects through the Employee just fine as well.

However, if I load employees with a query with left join fetch, like select e from Employee e left join fetch e.assignments, the associated Assignment/Project objects don't get loaded even though I can see the DB query is in fact doing the join and the data is coming back.

What is it about the above mappings that somehow doesn't interact with left join fetch properly?

I have used left join fetch in simpler @ElementCollection mappings with no problem so I'm wondering what the problem is.

Thanks!