Impacted versions:
* 11.0

Steps to reproduce:
* Create company A
* Create company B
* Admin user with both companies but currently in company B
* Create Demo user only with company B
* Create fiscal position A to company A
* Create fiscal position B to company B
* Create vendor with fiscal position A with company A and the same vendor with fiscal position B in company B
* Create product and add the vendor created before
* Set like **Create a draft purchase order**
* Logging with Demo user
* Create sale order with one sale order line with route **make to order**
* Validate sale order

Current behavior:
The purchase order created by sale order has the fiscal position **company A**

Expected behavior:
The purchase order must have fiscal position **company B**
addons/purchase/models/purchase.py
"""Return the datetime value to use as Schedule Date (``date_planned``) for
           PO Lines that correspond to the given product.seller_ids,
           when ordered at `date_order_str`.
           :param browse_record | False product: product.product, used to
               determine delivery delay thanks to the selected seller field (if False, default delay = 0)
           :param browse_record | False po: purchase.order, necessary only if
               the PO line is not yet attached to a PO.
           :rtype: datetime
           :return: desired Schedule Date for the PO line
        """
        date_order = po.date_order if po else self.order_id.date_order
        if date_order:
            return datetime.strptime(date_order, DEFAULT_SERVER_DATETIME_FORMAT) + relativedelta(days=seller.delay if seller else 0)
        else:
            return datetime.today() + relativedelta(days=seller.delay if seller else 0)
    @api.onchange('product_id')
    def onchange_product_id(self):
        result = {}
        if not self.product_id:
            return result
        
        self.date_planned = datetime.today().strftime(DEFAULT_SERVER_DATETIME_FORMAT)
        self.price_unit = self.product_qty = 0.0
        self.product_uom = self.product_id.uom_po_id or self.product_id.uom_id
        result['domain'] = {'product_uom': [('category_id', '=', self.product_id.uom_id.category_id.id)]}
        product_lang = self.product_id.with_context(
            lang=self.partner_id.lang,
            partner_id=self.partner_id.id,
        )
        self.name = product_lang.display_name