[13.0][FIX] account_banking_mandate: fixes singleton error when posting some records with mandate

account_banking_mandate/models/account_move.py

@@ -21,7 +21,8 @@ class AccountMove(models.Model):
     )
 
     def post(self):
-        self.line_ids.mandate_id = self.mandate_id
+        for record in self:
+            record.line_ids.write({"mandate_id": record.mandate_id})
         return super(AccountMove, self).post()
 
     @api.model