Email fetching is only working when 'Fetch on Auto-cron' is enabled, and when active it sends all fetched emails to the Admin Email address as new alerts.

You must set up a proper cron job or scheduled task -- enabling fetching alone does not start polling. See: https://docs.osticket.com/en/latest/GettingStarted/POP3-IMAPSettings.html. Disable the Auto-cron option once a real cron is configured to prevent conflicts. The emails being forwarded to the Admin are caused by the Admin Email being configured as a recipient for New Ticket Alerts -- review and adjust alert recipients in Admin Panel > Emails > Alerts & Notices. Full guide: https://docs.osticket.com/en/latest/Guides/AlertsGuide.html