For anyone having this issue:
Change the kms driver to the fake one by editing /boot/config.txt
change
dtoverlay=vc4-kms-v3dto
dtoverlay=vc4-fkms-v3dReboot & display will now switch on/off using
vcgencmd display_power 0 vcgencmd display_power 1I’ve not noticed any problems using the fake driver but I’m not using anything 3D (your results may be different)