unused packages aren't removed automatically

[nlany]

It seems that some automatically installed packages are not get removed automatically if they depend on each other (and no manually installed packages depend on or recommend any of them).

How to produce on Ubuntu 16.04 Desktop LTS:
1. Select "Tasks" ( predefined groups of packages for particular purpose) from packages view, then --> End-User-->"buntu desktop This task provides the Ubuntu Desktop environment. (1366)"
2. Mark entire group as automatically installed ( type "M" ). As group is not meta package, some packages in this group are shown as to be removed (package status is "idA").
3. Search for package "lightdm", which status is still "i A" ( but should be "idA" ), even though "lightdm" *is unused* now-- no manually installed package depends on or recommends "lightdm".

This bug prevents user to trim down ubuntu as needed. This bug exits no matter the "Install recommended packages automatically" is checked or not in preferences. And command "apt-get autoremove" doesn't remove such unused packages either, so this bug may affect apt-get too.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: aptitude 0.7.4-2ubuntu2
ProcVersionSignature: Ubuntu 4.4.0-124.148-generic 4.4.117
Uname: Linux 4.4.0-124-generic i686
NonfreeKernelModules: wl
ApportVersion: 2.20.1-0ubuntu2.17
Architecture: i386
CurrentDesktop: Unity
Date: Mon May 14 02:31:18 2018
InstallationDate: Installed on 2014-12-11 (1249 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release i386 (20140722.2)
SourcePackage: aptitude
UpgradeStatus: Upgraded to xenial on 2018-05-10 (2 days ago)