What kind of bug would make machine learning suddenly 40% worse at NetHack?

Members of the Legendary Computer Bugs Tribunal, honored guests, if I may have your attention? I would, humbly, submit a new contender for your esteemed judgment. You may or may not find it novel, you may even deign to call it a "bug," but I assure you, you will find it entertaining.

Consider NetHack. It is one of the all-time roguelike games, and I mean that in the more strict sense of that term. The content is procedurally generated, deaths are permanent, and the only thing you keep from game to game is your skill and knowledge. I do understand that the only thing two roguelike fans can agree on is how wrong the third roguelike fan is in their definition of roguelike, but, please, let us move on.

NetHack is great for machine learning…

Being a difficult game full of consequential choices and random challenges, as well as a "single-agent" game that can be generated and played at lightning speed on modern computers, NetHack is great for those working in machine learning—or imitation learning, actually, as detailed in Jens Tuyls' paper on how compute scaling affects single-agent game learning. Using Tuyls' model of expert NetHack behavior, Bartłomiej Cupiał and Maciej Wołczyk trained a neural network to play and improve itself using reinforcement learning.