AppEngine + Datastore + Objectify: Http Request returns inconsistent responses

Question

I am implementing AppEngine server as a backend for my Android application. I use Datastore, I query it via Objectify service and I use Endpoints that I call via URL.

I have an entity User with properties like this:

@Id
Long id;

/**
 * Name or nickname of the user
 */
public String name;

@Index
public String email;

@JsonIgnore
List<Key<User>> friends;

public User()
{
    devices = new ArrayList<String>();
    friendsWithKey = new ArrayList<Key<User>>();
}

public static User findRecordById(Long id)
{
   return ofy().load().type(User.class).id(id).now();
}


   @ApiMethod(name = "friends", httpMethod = "GET", path = "users/{userId}/friends")
public JSONObject getFriends(@Named("userId") String userId)
{
    User user = User.findRecordById(Long.parseLong(userId));

    JSONObject friendsObject = new JSONObject();
    JSONArray jsonArray = new JSONArray();
    JSONObject friend;
    List<User> userList = new ArrayList<User>();

    if (user.friendsWithKey != null)
    {
        for (Key<User> id : user.friendsWithKey)
        {
            friend = new JSONObject();
            User user1 = User.findRecordById(id.getId());
            userList.add(user1);
            friend.put("name", user1.name);
            friend.put("email", user1.email);
            friend.put("id", user1.id);user1.lastTimeOnline.getTime());
            jsonArray.add(friend);
        }
        friendsObject.put("friends", jsonArray);
    }
    return friendsObject;
}

It sometimes returns only a subset of friends. It is weird and I do not get where I could go wrong. If I get the User object from the DB, it already has a wrong List of Key values. But if I look into the database via console, I can see all of the users that ahve been added as friends. I reaally need to fix this bug. Please, help.

It is very strange because it only happens once in a while and is non-deterministic in every way.