We had a similar issue. I believe you'll have to raise a Service Request with Ariba and ask them to set the parameter Application.ACM.MinDaysCompletedTasksStayOnToDoPortlet to 0. This would make the completed task disappear from the To Do Portlet as soon as the task is complete as otherwise the default is to show tasks completed in the last 3 days.
this can be done via inspector
to do list contains the approve items