This issue is fixed in the version 4.8.1908 update of Windows Defender. After this update is applied, PowerShell files that are part of the Windows image are not changed, and the SFC tool no longer flags these files. Internet-connected computers that subscribe to the Windows Update channel automatically download and install this update.
To repair the Windows image files on computers that have been affected by this issue, use the DISM tool. To do this, open a Command Prompt window on the affected computer, and run the following commands:
dism /online /cleanup-image /restorehealth
sfc /scannow
If these commands fail and generate an error message that resembles "File not found," make sure that the Install.wim file is accessible, and then run the following commands:
DISM /Online /Cleanup-Image /RestoreHealth /Source:WIM:c:\install.wim:1 /LimitAccess
sfc /scannow
For more information about repair commands, see Repair a Windows image.