Email address exists in the list at Domains > example.com > Email Addresses > email@example.com > Forwarding tab even mail forwarding is disabled. The list cannot be completely cleared.
This is Plesk bug with ID #PPPM-11147 which is planned to be fixed in one of the future product updates.
This is a cosmetic issue which does not affect Plesk functionality and can be safely ignored. The mail forwarding is disabled for email address if the box Switch on mail forwarding is unchecked.
Currently, there is no available workaround for this issue.