And you followed this guide when disabling Defender periodic scanning? Does it re-enable itself only at the next reboot/restart or is doing so randomly? With 3rd party antivirus defender should be getting disabled with the exception of Windows Firewall due to it being used by Windows Updates. Can you post a screenshot of the message for review?
AND you’ve disabled tamper protection to disable scanning?
https://www.howtogeek.com/permanently-disable-microsoft-defender-on-windows-11/#:~:text=To%20turn%20off%20these%20settings,Yes”%20to%20confirm%20your%20decision.
SA