After encountering an issue where my custom Char field 'report_no' was getting lost when I saved new invoice line records in Odoo 15, I spent two hours troubleshooting the issue. Eventually, I discovered that the issue arose because the 'account.move' model contains two fields with the same relation to 'account.move.line', causing a conflict with my custom field. so I added the 'report_no' field in both one2many relation fields in the views 

    


    



Regards

Hi,

Try with the field attribute “ store”. 

Eg: 

class AccountMoveLineInherited(models.Model):
   _inherit = 'account.move.line'
   report_no = fields.Char('Report No', required=True, store= True)

If this is not works, try with the force_save = 1 in XML 

<field name=”report_no” force_save=”1”/>

Regards