It seems when you flag an email for the first time eventually the task will show under Tasks > To-Do List. When you flag the second email, it will show almost immediately under Tasks. However, in both cases if you navigate to Tasks and perform a search by any word to identify the recent added task it won't show results. And will not give results until you have Outlook restarted.
If the user has many tasks and tries to flag an email to create a new task, it will show under Tasks. However, you cannot perform a search to find it. The new task will not always show on top due custom order/filtering and from our perspective it seems the new task is indexed on the search engine when Outlook is restarted, not immediately.
Outlook Version 2006 (Build 13001.20384 Microsoft Store) Latest Version