We have Nintex workflow implemented in SharePoint Online.
If the workflow is completed, then there is no data available under Completed workflow.
But if the workflow is In progress then the workflow data is available under running workflow.
Also, this behavior is observed with items that are more than 1 month old.
Can anyone help here to understand what is exactly happening here?
Everything is normal.
SharePoint automaticaly delete details of workflow instances that is Terminated after 1 month (for maintenance purposes).
Unfortuntly you can't change this settings....
If you need to keep a history of your workflow actions, I recommend that you build your own history (with a Note field) and save it in your item.
Tips of the week :
you can still get all values of "Log to history" action. Every log are saved in a SharePoint list that is hidden, but it's a bit complicated to find your way around.
The list URL is : [URL Of Site]/Lists/Workflow History/AllItems.aspx