Package: gdm3 Version: 3.22.3-3+deb9u1 Followup-For: Bug #891599 This actually looks like it might be a bug in gnome-shell, and still present in 3.28. This silly-looking(?) bit of logic is present there in js/ui/status/network.js:
_sessionUpdated() { let sensitive = !Main.sessionMode.isLocked && !Main.sessionMode.isGreeter; this.menu.setSensitive(sensitive); }, I.e. it looks like it's hard coding things to not allow interaction from the greeter, no matter what permissions you might or might not have set.