A note to those who test kde apps building them from sources

if you encounter a problem with unfindable plugins like the one described here, the answer is simple: move fresh plugins to 'qt5' subfolder, e.g. /usr/lib/x86_64-linux-gnu/qt5/plugins instead of /usr/lib/x86_64-linux-gnu/plugins

Update: or pass -DKDE_INSTALL_USE_QT_SYS_PATHS=ON to cmake command (thanks to csslayer for pointing that out)