in mrp_prod_custom.py :

class MRPProduction(models.Model):    
	​_inherit = "mrp.production"    
	​_order = "is_priority_search desc"    
	​sale_id = fields.Many2one('sale.order', string="SO #", compute="_get_sale_id", store=True)
   sale_id_unlink = fields.Many2one('sale.order', string="SO #", related="sale_id")
    # Update from source if sale_id is NULL. For not auto generated MO.
    @api.depends('sale_line_id', 'origin')
    def _get_sale_id(self):
        for result in self:
            result.sale_id = None
            if result.sale_line_id:
                result.sale_id = result.sale_line_id.order_id
            elif result.origin:
                sale_id = self.env['sale.order'].sudo().search([('name', '=', result.origin)])
                result.sale_id = sale_id[0] if len(sale_id) else None
                if result.sale_id and result.sale_id.commitment_date:
                    # once get sale_line_id, udpate date_deadline from commitment_date in sale_order
                    result.date_deadline = result.sale_id.commitment_date - timedelta(days=2)
                    result.date_deadline_actual = result.sale_id.commitment_date
	​sale_mrp_production_ids = fields.One2many(comodel_name='mrp.production', string="SPK List", related="sale_id.mrp_production_ids")    
	​sale_date_order = fields.Datetime(string="Order Date", related="sale_id.date_order")    
	​sale_date_order_actual = fields.Datetime(string="Order Date", related="sale_id.date_order_actual")

so there's in acc_move_custom.xml: 

record model="ir.ui.view" id="flexypack_account_form">        
	​field name="name">flexypack.account.form	​field name="model">account.move	​
	​field name="inherit_id" ref="account.view_move_form"/>        
	​field name="arch" type="xml">            
	​	​data>                                
	​	​	​xpath expr="//field[@name='ref']" position="after">                    
	​	​	​	​field name="is_sales_document" invisible="1"/>                    
	​	​	​	​field name="paid_all_approval_approved" invisible="1"/>
	​	​	​	​field name="is_claimed_for_sale_id" invisible="1"/>                    
	​	​	​	​field name="is_referral_reward"/>                    
	​	​	​	​field name="production_ids" />                    
	​	​	​	​field name="production_names" />                    
	​	​	​	​!--  -->                    
	​	​	​	​field name="extra"/>                
	​	​	​/xpath>
	​	​	​xpath expr="//page[@id='invoice_tab']" position="before">
	​	​	​	​field name="return_sale_id" invisible="1"/>                    
	​	​	​	​page id="sales_out_invoice_line_ids_tab" string="Return Invoice Lines" attrs="{'invisible': ['|', ('type', '!=', 'out_refund'), ('return_sale_id', '=', False)]}">  	​	​	​	​	​field name="sales_out_invoice_line_ids">
	​	​	​	​	​	​tree string="Journal Items" default_order="sequence, date desc, move_name desc, id">                                
	​	​	​	​	​	​	​!-- Displayed fields -->                                
	​	​	​	​	​	​	​field name="sequence" widget="handle"/>                    
	​	​	​	​	​	​	​field name="move_name" invisible="1"/>
	​	​	​	​	​	​	​field name="product_id" optional="show" domain="['|', ('company_id', '=', False), ('company_id', '=', parent.company_id)]"/>                            	​	​	​	​	​/tree>                        
	​	​	​	​	​/field>                    
	​	​	​	​/page>                
	​	​	​/xpath>
	​	​	​xpath expr="//field[@name='invoice_line_ids']/tree/field[@name='product_id']" position="after">                                      
	​	​	​	​field name="production_id" string="No SPK"/>                              
	​	​	​	​field name="production_ids" />                    
	​	​	​	​field name="production_names" />                
	​	​	​/xpath>

in this acc_move_custom.py:

class AccountMove(models.Model):    
	​_inherit = 'account.move'    
	​notes = fields.Text(string='Notes')    
	​production_ids = fields.One2many(string='Manufacturing Orders', comodel_name='mrp.production',related='sale_id.mrp_production_ids')

class AccountMoveLineCustom(models.Model):   
	​_inherit = 'account.move.line'    
	​brand_id = fields.Many2one(comodel_name="partner.brand", string="Brand")    
	​variant_id = fields.Many2one(comodel_name="res.partner.brand.variant", string="Variant")
	​order_lines = fields.Many2many('sale.order.line','sale_order_line_invoice_rel','invoice_line_id', 'order_line_id', string='Order Lines', copy=False)        
	​acc_move_id = fields.Many2one('account.move')       
	​mrp_prod_ids = fields.Many2one('mrp.production')
	​production_ids = fields.Char(string='Manufacturing Orders',related='mrp_prod_ids.name')
	​# production_ids = fields.Char(string='Manufacturing Orders',related='mrp_prod_ids.sale_mrp_production_ids.name')    	​
stock_move_id = fields.Many2one("stock.move", string="Stock Move", domain="[('picking_id', '=', delivery_id)]")    
	​production_id = fields.Many2one(related="stock_move_id.created_production_id", string="Nomor SPK")    
	​fp_final_invoice_id = fields.Many2one('fp.final.invoice.line')    
	​production_names = fields.Many2one('fp_final_invoice_id.production_id', string="Nomor SPK", store=True)
	​# @api.depends('fp_final_invoice_id')    
	​# def _compute_production_id(self):    
	​#     for line in self:    
	​#         if line.fp_final_invoice_id:    
	​#             line.production_id = line.fp_final_invoice_id.production_id    
	​#         else:    
	​#             line.production_id = False    
	​# production_ids = fields.One2many(string='Manufacturing Orders', comodel_name='mrp.production',    
	​#                                  related='sale_id.mrp_production_ids')

I already tried to show the data of 'production_id', 'production_names', and 'production_ids', in:

xpath expr="//field[@name='ref']" position="after"> 

it's show, but when I add in: 

xpath expr="//field[@name='invoice_line_ids']/tree/field[@name='product_id']" 
position="after">

I'm tried get 'name' field from mrp.production, but the data won't show, it's empty.