If the user adds the "Invoiced Discount %" manually after 1 line was added, that is the Invoice Discount Amount that will take place in the whole invoice. No matter if you add new lines, that Invoice Discount Amount will be adjusting the percentage (%) using the new Invoice Total Amount, but keeping the calculated Invoice Discount Amount of the beginning.

Setting the Invoice Discount in the Customer Card wouldn't help, since we don't apply the same discount for that customer.

We need a way to set the Invoice Discount that applies to all lines, including new one.