When I create a custom faction+sovereign, the only custom I have so far, and go to the "Customize Opponents" screen in the last step of the game setup, the last faction is missing in the list. In this case it is Verga Reborn. This is not the case when there are no custom faction. I think this happens also to custom factions if you dont have the DLC, i.e. the last faction in the list is missing. I reported this bug a while back already. It would be nice if this got fixed eventually, because currently it 'costs' me a faction I payed for.