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.
<span style="color:#323232;">~$ sudo apt install kde-full
</span><span style="color:#323232;">Reading package lists... Done
</span><span style="color:#323232;">Building dependency tree... Done
</span><span style="color:#323232;">Reading state information... Done
</span><span style="color:#323232;">kde-full is already the newest version (5:147).
</span><span style="color:#323232;">0 upgraded, 0 newly installed, 0 to remove and 87 not upgraded.
</span>
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).