The Qt 5.9 packages provided by neon install a /etc/xdg/QtProject/qtlogging.ini which disables all debug output by default. This should not be.
Fwiw, I know of other distros that do similar (fedora is one), for arguably good reasons (minimize overly verbose debug-only items in logs)
It was deleted when we first spoke about it but in .deb packages when a configuration file is deleted it doesn't get deleted so I've added some code now to do that https://packaging.neon.kde.org/qt/qtbase.git/commit/?id=e7a32aead89b09852a19967d80eb194c5afca35a let's see if that works on next build. for testing you need libqt5core5a build17 which is the last version that still shipped it and upgrade from there. (The file came from Debian who stole it from Fedora. You can argue the merits of doing that vs fixing it in the apps but it's not suitable for Neon which where contributors are a big use case.)
> for arguably good reasons (minimize overly verbose debug-only items in logs) Well, randomly changing behavior with upgrades isn't a good idea. I also tried kdebugsettings which warned me of this situation but I couldn't figure out how to remedy it (how to have it setup a qtlogging.ini in my home that overrules the /etc/xdg one). Also, it does not just affect Plasma but also completely unexpectedly my Qt Enterprise install in /opt where I suddenly wouldn't get any debug output anymore. It also affects qDebug() not just qCDebug().