Why does the Task "created by" show the account as "SharePoint App" instead of the workflow initiator?
 

When the workflow is triggered with a conditional start or by a scheduled workflow, the Task will be created by the SharePointApp instead of the workflow initiator.

When Nintex triggers a workflow for a conditional start or scheduled, we are able to override the initiator value to be the 'triggering user'.
However, the actions taken by the workflow are still performed under the security context of the Nintex app users (SharePoint App) which uses the API to start the workflow.

https://community.nintex.com/t5/Nintex-for-Office-365/Workflow-started-by-conditional-start-runs-under-the-Nintex-App/td-p/38426
https://nintex.uservoice.com/forums/218291-3-nintex-workflow-for-office-365/suggestions/38767360-workflow-started-by-conditional-start-runs-under-s