Hi,

The issue occurs because the link between Purchase Orders and Bills in Odoo 17 is based on computed and non-stored fields, which don’t automatically update or propagate values.

To display the accounting_note from the purchase order on its related bills, there are two reliable methods:

    Use a related field — define accounting_note = fields.Char(related="purchase_id.accounting_note", store=True, readonly=True).

    This automatically displays the purchase note on the bill and updates if the purchase note changes.   

     class AccountMove(models.Model):

        _inherit = "account.move"

        accounting_note = fields.Char(related="purchase_id.accounting_note", store=True, readonly=True)

    Copy the value when the bill is created — override the bill’s create method to set accounting_note from the purchase order once during creation.

    This keeps the bill’s note fixed, even if the purchase order changes later.

    class AccountMove(models.Model):

        _inherit = "account.move"

        accounting_note = fields.Char(readonly=True)

        @api.model_create_multi

        def create(self, vals_list):

            records = super().create(vals_list)

            for move in records:

                if move.purchase_id and move.purchase_id.accounting_note:

                    move.accounting_note = move.purchase_id.accounting_note

            return records

In short, the related field is best for dynamic syncing, while copying on creation is ideal for keeping a static record.

Hope it helps