The issue is with werecreatures on a full moon. Most humans exposed to Western culture (likely all NetHack players) have heard of werewolves. I think it’s safe to say that everyone who has heard of werewolves knows they are most dangerous on a full moon. Even if you are a total NetHack beginner you know to avoid these monsters on the full moon. The game even helpfully reminds you of this information both by telling you about the full moon and by having a werewolf howl incessantly when it’s on the same level. However the game does not explicitly fill in the gap for you. It expects culture to do that.

The advantage of human common sense over machine learning models — at least when it comes to role-playing games — is that we carry around a ton of this cultural information. A model trained only on NetHack — not on broader culture or folklore/fairytales/mythology/fantasy — is simply not going to be aware of this link between full moons and specific monsters becoming more dangerous. So if it’s developed a fairly naïve strategy of just fighting or avoiding everything in its path based on a model of relative strength then it’s going to be tripped up when an outside event (the phase of the moon) upends that model.