Issue
Workflow Status column is blank on a list or library.
Resolution
This is expected behavior of the Workflow Auto Cleanup Timer Job. This job runs daily by default and removes any workflow instances that have been marked as 'Complete' for more than 60 days. A common work around is to create a new SharePoint column and update the column as a part of your to ensure that you maintain an appropriate status for your reference.
Additional Information
The Workflow Auto Cleanup Timer Job is a Job Definition that is included in SharePoint On-Premises versions by default. It is not recommended to disable this timer job.