I meanwhile ended up using the buster-backports kernel
5.4.0-0.bpo.3-amd64. It seems the only way to avoid the nasty bug having
to switch VT to get the monitor back on after blanking/power save. The
Intel X11-driver has other drawbacks.

While dealing with that problem quite often (also when upgrading 2
Lenovo-Tx20 laptops) I observed following with Ivy-Bridge and
Sandy-Bridge CPU's):

This problem manly happens when I first boot into another OS
and thereafter into Buster without power cycle. Happens i.e. when you
upgrade Stretch using a VT and finally reboot. Also happens when I save
the complete Buster partition for backup using i.e. Clonezilla or on my
PC where I have dual-boot with Stretch and Buster.

This problem seems to dissappear with time when further exclusively
booting Buster.

Could it be that in Buster the GPU is not reset/initialized correctly
and there are some settings remaining from previous boot/configuration?

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