I do not experience on KDE Neon. See this; you need to change something:
I made a change for the next version to not respect a system- or user-supplied QT_QUICK_CONTROLS_CONF environment variable in case that is the issue.
Here is our heuristic for determining light vs. dark in the System or System Fusion themes:
(palette().color(QPalette::Text).lightnessF() > 0.5f)
It is possible you are using a system palette that confuses that. That reads simply: if the text is more than half-gray (i.e. white, near white, or light) then use the dark theme palette. The problem you see is that the light theme’s palette is being used for the Qt Quick QML parts of the UI.
It is also possible the package you are using (not our build) did something weird to patch or break what we have done.