suspend display when in login manager login screen

Question

I have a laptop and a PC, both with Debian 10 installed. I am using slim login manager on both.

When I log out from the X server session on my PC, I get the slim login screen. After few seconds, the monitor automatically suspends/sleeps.

But when I log out from X on my laptop, the slim login screen stays forever, and laptop display never suspends.

I cannot find any difference between my laptop and PC. Where is this configured? How can I suspend laptop display, after I log out?

here is the relevant settings in the ServerLayout section of xorg.conf. These are same on both PC and laptop:

Option         "StandbyTime" "0"
Option         "SuspendTime" "0"
Option         "OffTime"     "0"
Option         "BlankTime"   "0"