I will have a look, but suspect that it is something with the firmware of the Pi4. There have been fundamental changes on the Pi4 regarding HDMI and quite a few issues. IIRC it works on other Pi models, e.g. Pi3 (but will have to test again).
If you look closely you can probably see that the screen on the Pi4 shuts off for a split second when the screensaver timeout you have set in the UI expires. Then the screen turns on again immediately. When you now run
xset -d :0 -q
you will get the result that DPMS is enabled and the DPMS state of the screen is “off” - just as it should be…
Edit: Seems to be a known issue… will have to check if older Pis are also affected.