laravel 先orderBY再groupby，导致分组后的排序不正确

 //联系过我的经纪人
            $appletChats=$this->AppletChat->orderBy('created_at','desc')->where([['user_id',$param['user_id']],['source',2]])
                ->get(['send_user','created_at'])->toArray();
            //  二维数组去重
            $res = array();
            foreach ($appletChats as $value){
                if (!empty($value['send_user'])) {
                    if (!isset($res[$value['send_user']])) {
                        $res[$value['send_user']] = $value;
                    }
                }
            }
            dd($res);