initializeEngine not called on iOS

I have a custom 'QQmlExtensionPlugin' which encapsulates, amon others, a custom ImageProvider.

I overrided the methods 'registerTypes' and 'initializeEngine' to register my custom types (the ImageProvider needs to be registered in the QmlEngine, thus i register it through the method 'initializeEngine'.

The application is developed to work on iOS and Windows. On Windows, both methods 'registerTypes' and 'initializeEngine' are called. But on iOS only 'registerTypes' is called.
(on iOS the plugin is correctly register through Q_IMPORT_PLUGIN).