Net 0 write off lines being generated on payments

As of a few months ago, we are encountering 'Write Off' line types being generated on some payments. These lines net to 0 (debit and credit to same account) but are creating numerous lines in the GL. After some investigation, it appears that nearly each time that this is occurring it is on a payment for an invoice which was previously fully paid and then had lines added to it. The write off line amounts are also always equal to a percentage of the new invoice value. For example, a $200 invoice is fully paid in May. A line is added to