Theming and scaling no longer works

After the latest update to my KDE Manjaro installation, themes no longer apply to Qt applications - they all default to the standard white colour scheme, and all scaling no longer applies. Qt applications are misaligned and GTK applications are really tiny.

I don’t even know where to begin solving this. Any ideas?

Applications that “follow the theme” but are not based on Qt or GTK work fine. Examples of this include Konsole, KeePassXC (with the old theming set), Firefox, etc.

Does this also happen in a newly created user?

I.E.: Create Ishayu2, log in there and see if it happens there as well…