You may have seen that you can either swipe a mail or tap the archive button in Outlook to archive mail. This will ask to create an Archive folder the first time you use this. This creates a normal subfolder which mail is “archived” to. This does not move the mail to the Online Archive. If you missed the initial request to create the Archive folder it will then use the last subfolder to “Archive” mails to. Users may think they are archiving mails when they are not.
You can set/view the folder location here: Settings > account name>Advanced Settings> System Folders