AppState not persisting even when its Persist is set true

I have this AppState variable which is a list and Persist is set to true as you can see in the screenshot below. But it is not persisting on app restart.

This is the custom code that i'm using to save it:

FFAppState().update(() {

          FFAppState().downloadedSounds.add(soundRef);

          currentDownloadStatus.downloadProgress = 0;

        });

So is there something missing in my code that should force it to persist? or is this some bug?

Thanks