The specifics of the calculation issue are that versions of InvoicePlane prior to 1.5.10 calculate the taxable amount from item quantity * price regardless of discounts. The net effect is that all invoices where a discount was applied to a taxable item produced an amount due that contained too much tax. (This bug should not affect invoices or quotes where a discount was not given).
In some cases, not only was too much tax collected, but because the way the discounts were applied, the over all amount due on the invoice was less money than it should have been, which means too much tax was collected and not enough money was collected on the total invoice.
In any given invoice, the difference in the amount of tax collected versus what should have been collected will be the taxable amount on the discount. (If you have a 7% tax rate, and you discount an item $100, you will have collected $7 extra tax).
Depending on your country, this may mean you have to file an amendment with your tax authority, and it may also mean you owe customers refunds for the over collection of taxes. Check with a tax professional to find out how you should proceed.