The appraisal due task is generated from the appraisal logbook.
If this has been marked out of use, the tasks will not be viewable within the tasks section. However, will still show any outstanding tasks before the logbook was marked out of use on the recap emails.
To mark the tasks as completed, the logbook needs to be marked as in use, and the tasks for the appraisal will then be shown in the Tasks tab.