GraphQL mutation updates _version flag but not the values I'm trying to update

I'm using AWS Amplify and AppSync on my application in Android Studio. Getting and creating items works fine but the update mutation doesn't seem to work. Here is my code:

Amplify.API.query(
            ModelMutation.update(d),
            { Log.i("Amplify", "Aggiornato con successo")
                Constants.getInstance().isQueryEnded = true}
        ) { error: ApiException? -> Log.e("Amplify", "Query failure", error) }

It actually prints the "Update succeeded" log ang updates the _version flag of the item but doesn't update the values that i need to update.