In https://doc.qt.io/qt-5/qqmlengine.html#QML_SINGLETON

This behavior can be overridden by calling qmlRegisterSingletonType() with a specific factory function or qmlRegisterSingletonInstance() with a specific instance for the same class and the same type namespace and version.

But calling qmlRegisterSingletonInstance causes all other QML_ELEMENT to not be available. I've attached an example application to demonstrate the issue. The example works if main.cpp:16 is commented.