I do. But that argument could be applied to a number of other things in ADOM as well, yet running into undead did not spoil the surprise when I discovered the CoC graveyard, running into blue dragons did not spoil the surprise when I discovered the blue dragon caves, running into assassins didn't spoil the assassin's guild... the list goes on. I don't believe running into animated trees in the forests where I'd expect them would have subtracted from the surprise of discovering them far underground where I wouldn't expect them.
If it helps, the level limit for the encounter could be somewhat higher, perhaps 20 or 25. That would make a new player managing to get to that level without having already discovered the animated forest very rare.