Hello, I had #849265 reported against adduser for this problem that exists in d-i's user-setup. It also exists in adduser if the --add_extra_groups flags is used (which is why I didn't reassign/merge), user-setup is adding this groups directly.
See the bug log for 849265 [1], as well as my correspondence with pkg-systemd [2]. Thanks and regards Afif 1. http://bugs.debian.org/849265 2. http://lists.alioth.debian.org/pipermail/pkg-systemd-maintainers/2017-January/013684.html -- Afif Elghraoui | عفيف الغراوي http://afif.ghraoui.name