The xcb integration sets the WM_CLASS to
[argv0BaseName(), QCoreApplication::applicationName()] (if one doesn't pass -name and doesn't set the RESOURCE_NAME environment variable) (see QXcbIntegration::wmClass()).

Qt/Wayland sets the app ID to QGuiApplication::desktopFileName() if that is set, or otherwise to a reverse dns identifier based on organization domain and applicationFilePath (see QWaylandWindow::initWindow()).

For Linux .desktop files, when running under wayland it looks like the Wayland App ID is used as a wmclass to match against the StartupWMClass, but that doesn't match the WM_CLASS that is used when running under X11/XCB.