Consider the following scenario:
In SharePoint Online or SharePoint Server, you have a workflow that uses the SharePoint 2013 Workflow Platform Type.
The workflow uses the Assign a Task or Start a task action.
However, after the workflow is completed, the completed task isn’t deleted as expected.
This is the expected behavior for workflows that use the SharePoint 2013 Workflow Platform Type. Completed workflow tasks aren't deleted. Incomplete workflow tasks are deleted by default unless you change the PreserveIncompleteTasks setting to Yes for the properties of the task in SharePoint Designer 2013. Workflows that use the SharePoint 2010 Workflow Platform Type delete both completed and incomplete tasks by default.
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