Issue Description
Why does some emails do not appear as an 'Email Filter' option in an Email/Email Link Performance Report?
https://docs.marketo.com/display/public/DOCS/Filter+Assets+in+an+Email+Report
Issue Resolution
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."
https://docs.marketo.com/pages/releaseview.action?pageId=2359491
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.
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