https://wccftech.com/microsoft-fixes-windows-defender-scans-broke/

An update for Windows Defender broke both the Quick and Full antivirus scans, with the WD only scanning for about 40 files and confirming that “no current threats” were discovered on the device.

“Microsoft Defender AV employs real-time scanning of endpoints, which was not impacted by this update,” the company had said in response to the issue. “Only manual or scheduled scans conducted by administrators were temporarily impacted and we are working to resolve it.”