EF1 Navigation Properties Not Working?

My entity model was generated from the existing database. There is a many-to-many junction table picked up and hidden by EF.

The relationship is definitely working because this query returns 2 users as expected.

public IQueryable<User> FindUsersByGroupID(int group_id)
{
return db.Users.Where(u => u.Groups.Any(g => g.Group_ID == group_id));
}

But when locating a user that is part of the above result set the Groups navigation property count is 0. I shouldn't have to explicitly join.. right?

public User FindUserByID(int id)
{
return db.Users.First(u => u.User_ID == id);
}