Hi Alexander, Eben, and everyone in list,
Thank you for your replies.
The Debian 12 is running in VirtulBox 7.0.20. And I only
assigned 2048 MB RAM to it.
I'm pretty sure that I have setup a big enough SWAP of around
8 GB when installing Debian 12.
When I click "Sleep" button in Windows running in VirtualBox
6.x, the virtual machine will light the screen up again once the
screen is off. However, in this case, Debian 12 doesn't light
the screen up nor power-off the virtual machine. So the virtual
machine goes into a state that OS is not running, power is still
on. And I have no way to wake the OS up. All I can do is to
fouce power-off and re-start it, just to found all running tasks
gone.
I don't use VirtualBox and I think this is VirtualBox issue not
Debian issue.