Based on tests in our labs, this problem doesn't affect Windows 10 v. 1909. It's therefore suggested to update the Windows to the last available version in order to solve this issue.

Blanking the display during a remote NoMachine session on Windows 10 is no longer possible since Microsoft update KB4501375 version 1903. This issue does not occur on Windows 10 previous versions, e.g. 1809.

Ref.
https://support.microsoft.com/en-au/help/4501375/windows-10-update-kb4501375
https://docs.microsoft.com/en-us/windows/release-information/status-windows-10-1903