Yes, I had many cases re-emerging factions becoming strong again, but I don't think their behavior in startpos files has something to do with it. It's only a variable that influnces the faction's behaviour in the first years. But the faction's behaviour is very changable and depends a lot on situation in the game. Most of the factions, set as expansionist or not, will attack you or any other fation if there is a good opportunity. I've seen many, many times in my campaigns, Almohads, declared as peaceful nation in the startpos file, becoming poweful and aggresive neighbour.