This doesn't seem like a problem specific to nircmd as it occurs with other software to turn off the monitor such as Dark Tool - Dark Tool was suggested in this article https://www.maketecheasier.com/turn-off-windows-screen/ along with other tools some of which I coudn't get to work but I think the same problem would have occured even if I could. Another question has referred to a similar problem when using both nircmd and other third party software: Monitor will not stay off. The solution for this individual was to turn off ForceAutoLogon in the registry settings - but for me this setting is not on.
To describe the problem: whenever I run 'nircmd monitor off' the monitor screen turns off but then turns back on after a few seconds. I have tried restarting the PC in safe-mode but running 'nircmd monitor off' still results in the monitor turning off and turning back on again in a few seconds. I have tried to uninstall various programs that I thought might be causing the problem, like anti-virus software, as well as trying using a different monitor but the same thing still occurs in both cases.
This problem doesn't seem to be occuring for everyone who runs 'nircmd monitor off' so does anyone have any ideas what might be turning the PC back on for me?