dist-upgrade wants to remove kubuntu_desktop

Question

I've been running ubuntu (18.04) with kde (kubuntu_desktop) on my computer for years. As of recently (at some point after upgrading to bionic), whenever I try to issue an apt-get update / apt-get dist-upgrade, it will remove kubuntu_desktop along with a lot of other packages, most of the kde-related. I will post the output at the bottom.

My questions are i) how does apt decide, what is going to be REMOVED ? My impression was that this should mainly affect packages that have been installed as requriments of other packages and became obsolete because the requiring package has been removed. And old kernels, of course. In my case, I have EXPLICITLY installed kubuntu_desktop. ii) how can I prevent kubuntu_desktop from being removed?

I ran the process once, saw that kde/plasma desktop was gone, re-installed it manually (apt-get install kubuntu-desktop), but this did not change anything. With the next dist-upgrade, kubuntu_desktop remains on the list of packages to be removed.

Here is the full output:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following packages were automatically installed and are no longer required:
  cryfs fonts-hack fonts-hack-ttf libaribb24-0 libbasicusageenvironment1 libcolorcorrect5 libcrypto++6 libdvbpsi10 libebml4v5 libgps23 libgroupsock8 libkate1 libkwinglutils11
  libkwinxrenderutils11 liblivemedia62 libmatroska6v5 libmicrodns0 libopenmpt-modplug1 libplacebo4 libqalculate6 libqalculate6-data libresid-builder0c2a libsdl-image1.2 libsidplay2
  libtaskmanager6 libusageenvironment3 libva-wayland2 libvulkan1 libxcb-xv0 plasma-integration pulseaudio-module-gconf qml-module-org-kde-kconfig qml-module-org-kde-kholidays
  qml-module-org-kde-qqc2desktopstyle qml-module-qtwebengine vlc-bin vlc-data vlc-l10n vlc-plugin-base vlc-plugin-notify vlc-plugin-qt vlc-plugin-samba vlc-plugin-skins2
  vlc-plugin-video-splitter vlc-plugin-visualization
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  bluedevil kate kde-config-screenlocker kinfocenter kscreen kubuntu-desktop kwin kwin-addons kwin-common kwin-x11 libkf5waylandserver5 libkscreenlocker5 libkwin4-effect-builtins1
  libkwineffects11 milou plasma-dataengines-addons plasma-desktop plasma-framework plasma-nm plasma-pa plasma-vault plasma-wallpapers-addons plasma-widgets-addons plasma-workspace
  print-manager sddm-theme-breeze vlc vlc-plugin-video-output
The following packages have been kept back:
  libegl1-mesa libgbm1 libwayland-egl1-mesa
0 upgraded, 0 newly installed, 28 to remove and 3 not upgraded.
After this operation, 95.4 MB disk space will be freed.
Do you want to continue? [Y/n] n

By request, here is my output of "sudo apt-cache policy apt"

apt:
  Installed: 1.6.3ubuntu0.1
  Candidate: 1.6.3ubuntu0.1
  Version table:
 *** 1.6.3ubuntu0.1 500
        500 http://de.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     1.6.1 500
        500 http://de.archive.ubuntu.com/ubuntu bionic/main amd64 Packages

And the output of "ls /usr/share/xsessions" is

plasma.desktop