Delegated for former employee expense report

If an employee leaves our company and the expense report is being rejected, then a delegate should correct the expense report and submit it again, but the delegate is not able to find the former employee in the drop-down menu in Expense.

But we face the issue - on some former employees - that even though the Manage delegation is setup, it is not possible to see the former employee in the drop-down list in expenses. How can we solve/fix this?