Why are departed employees appearing in my Plan Employee Salaries grid?

Access to and grouping of employees is ahcieved by using an organizational hierarchy that is sourced from iVantage.  When employees depart, their organization in iVantage for their current record is blank.  However, the business representatives of the project team requested that these employees not be removed from their last location n the hierarchy until the 2nd year after their departure.  This was so administrators could use their same employee-based filters and look back on how their working plan looked earlier in the current fiscal year and in the previous fiscal year in reports.  After an employee has been gone for 2 years the are moved to the Unknown grouping in the hierarchy:

Below is a sample of the hierarchy:

Employee-based Organizational Hierarchy