Microsoft has resolved a known issue that caused Windows to display false alerts saying Defender Antivirus was turned off. According to BleepingComputer, the incorrect warnings appeared after users installed recent updates. The alerts were misleading, as Defender Antivirus was not actually disabled.
With the fix now in place, affected users should no longer see the erroneous warning. The report does not specify any additional troubleshooting steps beyond Microsoft's resolution. As with similar known issues, installing the latest updates is the standard way to ensure the fix is applied.