DPMS does not switch off the monitor back light on Raspberry Pi 4

'xset dpms force off' blanks the screen but does not switch off the HDMI (LCD) monitor backlight.

'xset -q' shows DPMS is enabled.

'tvservice -o' does switch off the monitor.

No warnings/errors related to dpms in /var/log/XOrg.0.log

Not sure if this is a xorg, kernel, libxcb-dpms0 or other issue.

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: xorg 1:7.7+19ubuntu12
ProcVersionSignature: User Name 5.3.0-1012.14-raspi2 5.3.7
Uname: Linux 5.3.0-1012-raspi2 aarch64
ApportVersion: 2.20.11-0ubuntu8.2
Architecture: arm64
BootLog: Error: [Errno 13] Permission denied: '/var/log/boot.log'
CompositorRunning: None
CurrentDesktop: XFCE
Date: Fri Nov 15 18:33:52 2019
DistUpgraded: Fresh install
DistroCodename: eoan
DistroVariant: ubuntu
ExtraDebuggingInterest: Yes
GraphicsCard:

ProcKernelCmdLine: coherent_pool=1M 8250.nr_uarts=1 cma=64M cma=256M video=HDMI-A-1:1920x1080@60 smsc95xx.macaddr=DC:A6:32:17:19:0B vc_mem.mem_base=0x3ec00000 vc_mem.mem_size=0x40000000 net.ifnames=0 dwc_otg.lpm_enable=0 console=ttyS0,115200 console=tty1 root=LABEL=writable rootfstype=ext4 elevator=deadline rootwait quiet splash
SourcePackage: xorg
Symptom: display
UpgradeStatus: No upgrade log present (probably fresh install)
version.compiz: compiz N/A
version.libdrm2: libdrm2 2.4.99-1ubuntu1
version.libgl1-mesa-dri: libgl1-mesa-dri 19.2.1-1ubuntu1
version.libgl1-mesa-glx: libgl1-mesa-glx N/A
version.xserver-xorg-core: xserver-xorg-core 2:1.20.5+git20191008-0ubuntu1
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev N/A
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:19.0.1-1ubuntu1
version.xserver-xorg-video-intel: xserver-xorg-video-intel N/A
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.16-1