Join Table di Controller Tidak Bisa dengan Method Join yang Sama

dibawah ini kodingan controller untuk halaman paybill register

$this->db->select
            (
            "{$this->db->dbprefix('purchases_paybill_register')}.id as id, 
            DATE_FORMAT(real_paydate, '%Y-%m-%d') as real_pay, 
            reference_no, 
            branch,
            supplier,
            {$this->db->dbprefix('warehouses')}.code as name,
            GROUP_CONCAT(CONCAT({$this->db->dbprefix('purchase_items_paybill_register')}.product_name), ' ' SEPARATOR '\n') as iname,
            currency,
            invoice_no,
            date_invoice,
            tax_invoice_date,
            tax_invoice_no,
            voucher_no,
            bank_ref,
            actual_value, 
            total_pay,  
            payment_info, 
            status_transfer, 
            myob",
            false)
            ->from('purchases_paybill_register')            
            ->join('warehouses', 'warehouses.id=purchases_paybill_register.warehouse_id', 'left')
            ->join('purchase_items_paybill_register', 'purchase_items_paybill_register.purchase_id=purchases_paybill_register.id', 'left')
            ->group_by('purchases_paybill_register.id');

kodingan diatas saya ingin join data table 'purchase_items_paybill_register' dengan 'purchases_paybill_register' untuk mengambil multiple product dan jadi 1 row. Masalahnya adalah kodingan diatas tidak berfungsi.

Tetapi ketika saya join dengan table lain untuk mengambil data produk justru bisa. Sebagai contoh dibawah

$this->db->select
            (
            "{$this->db->dbprefix('purchases_paybill_register')}.id as id, 
            DATE_FORMAT(real_paydate, '%Y-%m-%d') as real_pay, 
            reference_no, 
            branch,
            supplier,
            {$this->db->dbprefix('warehouses')}.code as name,
            GROUP_CONCAT(CONCAT({$this->db->dbprefix('inventory_items')}.product_name), ' ' SEPARATOR '\n') as iname,
            currency,
            invoice_no,
            date_invoice,
            tax_invoice_date,
            tax_invoice_no,
            voucher_no,
            bank_ref,
            actual_value, 
            total_pay,  
            payment_info, 
            status_transfer, 
            myob",
            false)
            ->from('purchases_paybill_register')            
            ->join('warehouses', 'warehouses.id=purchases_paybill_register.warehouse_id', 'left')
            ->join('inventory_items', 'inventory_items.purchase_id=purchases_paybill_register.id', 'left')
            ->group_by('purchases_paybill_register.id');

diatas saya join data ke ke 'inventory_items' dan kodingan berfungsi dengan baik. Saya ingin join ke 'purchase_items_paybill_register' bukan 'inventory_items'. Saya sudah menerapkan logika yang sama tapi kenapa kodingan atas tidak bisa, tapi yang bawah bisa?

Apakah saya ada kesalahan atau kekurangan?

Terima kasih.