When using loader with custom qml types those are not freed when unloading

Using Loader to load qml file where custom types are being used (registered with 

qmlRegisterSingletonType) from separate qml plugin and then unloading (setting source = "") loaded qml file, those resources are not being freed as there still exists refCount > = 1 when calling 

QQmlMetaType::freeUnusedTypesAndCaches().

Use provided example:

1.  Press Load/Unload to trigger loading of qml file which will import separate qml plugin

2. Unload qml file by pressing again Load/Unload

3. Press gc to trigger garbage collection

4. Press free unused button in order to start cleaning unused types (which the ones loaded from separate qml file should now be)