Running Xfce4 on Raspberry Pi 400, was using FKMS video driver before the 22.06 update, but got the well known very bad mouse lag and had to switch over to KMS.
After that HDMI display sleep is not working.
With DPMS enabled, any screensaver/power management sleep mode makes the display go blank, lose video signal and eventually go to sleep. Immediately when the display would go to sleep, the video driver wakes up again. Seems like there is some new input event that triggers a wakeup when the display goes to sleep.
There is no observable difference between forcing different sleep modes with
xset dpms force (standby|suspend|off)
With DPMS disabled, sleep modes just blanks display with backlight on, as expected, because no DPMS signalling is asking it to power down.
Going back to the FKMS driver, DPMS sleep works as expected, but that’s not really an option given the severe performance issues.