It seems that the AI is unable to form Romania. I've tried multiple times to coax it into doing so by vassal feeding Moldavia or Wallachia and releasing them but it just won't do it. I suspect it may have to do with Maros having Hungarian culture and not Romanian. Since the ai rarely does culture conversions, it never happens. Maybe the culture in maros should be set to Romanian to assist it in doing so? If it's not that then maybe something else is preventing it from forming Romania?