Describe the bug
Issue noted on a Raspberry Pi 4, running Buster using both the stable kernel and the current bleeding edge.
The setting in /boot/config.txt "hdmi_blanking=1 is set. The X dpms settings are active. The screen blanks after the defined timeout but the backlight stays on.
This issue does not occur on a respberry pi 3b+ with the same SD card (this was tested prior to upgrading the kerne - so only on the current stable kernell)
This occurs on an ASUS HDMI monitor. Have no other display devices to test with.

To reproduce
Configure /boot/config.txt with hdmi_blanking=1.
In /etc/xdg/lxsession/LXDE-pi/autostart add:
point-rpi
@xset s 0 0
@xset s noblank
@xset s noexpose
@xset dpms 0 0 300

Expected behaviour
Screen blanks after 5 miniutes. Backlight switches off when screen receives no HDMI signal.

Actual behaviour
On Pi 3b+, after blanking the screen briefly displays blue with backlight, then switches off (which is fine). On Pi 4 it blanks completely, then the backlight switches on (which is no so good).

System
Pi 4.
Debian 10.0 Kernel4.19.50-v7l+ #895 and
Debian 10.0 Kernel 4.19.57-v7l+ #1244

Logs
Nothing logged in Xorg.0.log.

Additional context
Happy to try any suggested troubleshooting.