I see I am coming to this thread a bit late, but for what it is worth,
here is some additional information.
But first, thanks to Udo Richter for suggesting the workaround of
installing linux-image-5.4.0-0.bpo.2-amd64, in other words backporting the
testing/bullseye kernel into buster. On my laptop, this fixed the
intermittent problem restoring blanked screen. It also seems to have
introduced a new problem where the computer occasionally does not blank
or lock when it should, but that is far better than occasionally losing
work-in-progress due to inability to restore the screen.
However, this does not seem to simply be a case of buster's default 4.19.0
kernel being buggy. I say this, because I tried the backport version of
4.19.0 in stretch. On my laptop, screen blanking and locking work just
fine in stretch, regardless whether it is the default 4.9.0 or the
backported 4.19.0 kernel. Apparently, something besides the kernel
changed between stretch and buster.
Some background info: I have been testing with the xfce desktop on a
Panasonic CF-19 laptop. Intel Corporation Xeon E3-1200 v3/4th Gen Core
Processor Integrated Graphics Controller (rev 06)