i have tried to download an older version from here: http://snapshot.debian.org/package/gnome-screenshot/3.18.0-1/#gnome-screenshot_3.18.0-1:2b:b1
and run just the binary of ./gnome-screenshot -a but it yields the same faulty result. So probably the issue is in the linked libs, in gnome-shell 3.20.xxx or somewhere else... btw. sorry for all the previous typos. cheers P.