QQmlExtensionPlugin::initializeEngine() called for the first QQmlEngine instance only

The initializeEngine() function of an extension plugin is called only once, for the engine that first creates a component that imports the extension. If a component later imports the same extension in another engine, the function will not be called.

It is OK for the registerTypes() function to be called only once, but initializeEngine() should be called on each engine instance that imports the extension.