When using Nintex Workflow for Office 365 "Create List Item" action and the workflow is configured to start on a conditioned (Conditional Start Workflow), the created item will have the "Created By" item property as SharePoint App instead of the workflow initiator. 

This is currently a platform limitation based on how the "Conditional Start Workflows" are configured to start and run. The "Conditional Start Workflow" will have SharePoint App as the workflow initiator.