Dunning letter with credit item

Good morning,

We have a customer who has a credit that was posted 400 days past and an invoice that is just 23 days past due. When we generate the dunning letter by days for items that are 60 days past due, both the 23-day invoice and the 400-day credit appear on the letter. It's clear that the letter is generating because there is an item that is over 60 days past due, but the credit item is not excluded when the option for "Exclude Credit Items" is checked in the Dunning Method set up configuration.

We would like to have the letter not generate based on the age of the credit item. Have we missed something obvious?  This is on version 9.1.