Indeed it looks like a bug. I finally found a solution, though. Check the main post edit for details.

You are right, I should have made this clear. I am not on Nvidia, I am using an old Thinkpad on Intel Haswell.

I’m glad to know my problem isn’t completely new. I’ll look into it further online. If you ever find a link to a report of a similar issue, I would be happy to see!

Edit: I found this link, the issue reported appears to be very similar to what I’m seeing here https://libreddit.tux.pizza/r/kde/comments/jhqbnz/kde_plasma_rendering_problem_black_squares/

Edit 2: I finally solved my problem! It was indeed an old problem already reported somewhere.

I agree with that. I suspect you might be right. SDDM (Breeze) is also weird with transparency. However, I just installed materia-kde but unfortunately the problem persisted (screenshot attached). Before that, I ran apt purge kde* plasma* libkf* and apt install kde-full. That too didn’t solve my problem.

Tasksel seems to be correctly set,

~$ tasksel --list-tasks
i desktop       Debian desktop environment
u gnome-desktop GNOME
u xfce-desktop  Xfce
u gnome-flashback-desktop       GNOME Flashback
i kde-desktop   KDE Plasma
u cinnamon-desktop      Cinnamon
u mate-desktop  MATE
u lxde-desktop  LXDE
u lxqt-desktop  LXQt
u web-server    web server
u ssh-server    SSH server
i laptop        laptop

I tried apt reinstall kde-full, but unfortunately nothing changed.

Thanks for the tip! However, I tried apt reinstall kde-full and apt --fix-broken install, but no packages were installed and (unsurprisingly) the problem still persists.

Yeah, I tried installing kde-full but it didn’t solve my problem.

Sadly I am not using BTRFS for my root directory on this specific system. If I end up deciding to reinstall, I will definitely go back to BTRFS to avoid such problems.

Debian actually has a KDE group named kde-full. I reinstalled it but the issue persists, which was honestly surprising to me.

~$ sudo apt install kde-full
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
kde-full is already the newest version (5:147).
0 upgraded, 0 newly installed, 0 to remove and 87 not upgraded.

The new user idea was really clever, thanks for the suggestion! I will try that now and see.

Edit: the new user also presents the same problem. Actually, it makes sense, since SDDM is affected as well (I should have mentioned that before).