The backup destination configuration files in /var/cpanel/backups/*back_destination are manually modified and an absolute path is added for the value of "Backup Directory". After making the changes, the "Save Destination" button becomes greyed out and no changes to the configuration interface are detected or saved.
Absolute paths are not allowed for the value of "Backup Directory", relative paths need to be used instead. You can refer to this link for more information:
The configuration file in /var/cpanel/backups/*backup_destination must be manually edited and the absolute path must be replaced with a relative path to make the interface editable again.