Oops, I had already uploaded the eog and evince changes before you edited. Could you please report a bug against the Debian gtk3-nocsd package?
Could you maybe have gtk3-nocsd check the environment variables? These are what I have on my system. (We’ll also want to do the same for Pop!_OS. I believe they are just ‘pop’ instead of ‘ubuntu’.)
XDG_SESSION_DESKTOP=ubuntu
XDG_CURRENT_DESKTOP=ubuntu:GNOME