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).