Issue
Solution
The reason why some emails do not appear as an 'Email Filter' option in an Email/Email Link Performance report is because the emails were used in an A/B test. Similar to the product behaviour illustrated below, when adding an A/B test, the selected email will not be directly selectable or appear as an Email Filter option in an Email/Email Link Performance report.
"When adding an A/B test, the selected email will no longer be available for use in any other program."
Add An A/B Test
In order to report on the A/B test emails that do not appear in the Email Filter option, the parent program can be selected.