Why Does the Workflow Status Disappear in SharePoint 2010?
Symptoms
In SharePoint 2010, the workflow status links for a work item disappear over time.
Cause
By default, Microsoft SharePoint Server 2010 runs a daily Workflow Auto Cleanup job to permanently delete workflow instances and related task entries that still exist 60 days after a workflow is completed or canceled. Workflow history items themselves are not deleted, but the entry point to view them on the status page for a particular instance of a workflow will no longer be available.