This problem reappeared after some other changes. So, an updated Debian and an updated Linux were not the (sole) resolution of this problem.
What is working for me now was a work-around suggested in a thread elsewhere at Framework community, and in a couple of Freedesktop bug reports 2352 2354.
The work-around is:
Add to the Linux command line, the option amdgpu.sg_display=0.
On Debian GNU+Linux, this is easiest to do by editing the ‘/etc/default/grub’ configuration variable GRUB_CMDLINE_LINUX_DEFAULT, to include that option.
(The option is apparently referring to AMD’s “Scatter/Gather” processor feature, which they’ve recently turned on by default?)
With that option disabled, now the display can turn off (e.g. when the computer is suspended), and when it comes back on, the display is the same as when it turned off, no garbage artifacts like before.
I don’t know what the actual bug is. Nor do I know what I’m losing by using this work-around (maybe GPU performance?). I hope this bug is fixed, but don’t know when it’ll be fixed.