Post the rest of your code, the contents of vals, and the types of each of the accounts you are using, there could be many things and readers can not guess from the information you have shared.

Thanks for pointing out. This is the code snippet base on the query above.

def create_bill_invoiced(self, invoice_id, agent, vendor_name, amount=20000):
    # agent is a custom model
  
    # Create a service-type product template
    product = self.env['product.template'].create({
        'name': f"Bill from {vendor_name}",
        'list_price': amount,
        'type': 'service',
        'sale_ok': False,
        'purchase_ok': True,
        'company_id': agent.company_id.id,
        'taxes_id': [],
        'supplier_taxes_id': [],
    })

    variant = product.product_variant_id
    if not variant:
        return False

    # Build a single invoice line for the variant
    invoice_line = (0, 0, {
        'product_id': variant.id,
        'name': variant.name,
        'quantity': 1,
        'price_unit': amount,
        'account_id': self.get_product_account(product_id=variant.id, company_id=agent.company_id.id),
    })

    # Create the bill (vendor invoice)
    bill_invoiced = self.env['account.move'].create({
        'move_type': 'in_invoice',
        'partner_id': agent.portal_user.partner_id.id,
        'invoice_date': fields.Date.today(),
        'invoice_date_due': fields.Date.today() + timedelta(days=5),
        'invoice_line_ids': [invoice_line],
        'ref': f"Related Invoice ID: {invoice_id}",
    })

    bill_invoiced.action_post()
    return bill_invoiced

The

bill_invoiced = self.env['account.move'].create

 actually created a record and it is Posted automatically by

bill_invoiced.action_post()

.However the amount due is 0. I checked the other values and it seems the amount residual is also 0. Other values such as Total amount and subtotals are correct. I tried to manually reset the record to draft and post it again. It works, but I should be able to do this via code.