For anyone having this issue:

Change the kms driver to the fake one by editing /boot/config.txt

change

dtoverlay=vc4-kms-v3d

to

dtoverlay=vc4-fkms-v3d

Reboot & display will now switch on/off using

vcgencmd display_power 0
vcgencmd display_power 1

I’ve not noticed any problems using the fake driver but I’m not using anything 3D (your results may be different)