When using the Task action in Nintex Workflow for Office 365 with the "Auto-complete task" setting being activated, the user finds the "task date completed" is the same date when the task created, this can be confusing as to when the Task was actually completed. 

Make sure to get the information on the task last date modified from the task itself (via workflow task list) and not from the item related to the task. 

Use the Query List action by supplying the Task ID from the Task action output, to get the information details of the completed Task.