Plasma-Integration opens kdeglobals through KSharedConfig, so when a KCM then instead uses KConfig() and syncs it, krdb won't know it and write outdated values.
The fonts KCM, for instance, does that and we ended up not saving the new font hinting settings.

5.17
Opened fonts kcm, changed font hinting, checked xrdb -q | grep Xft to see it matched what I had configured