This seems like fruit of the poisonous tree. They didn't monitor their training environments, so I bet the reward hacking just got incorporated into their training corpus. In other words, agents solved some tasks, but not quite as intended, because of reward hacking. Instead of discarding that data, they included it in the training data for later checkpoints. And once that signal is reinforced, it happens more often, so the more it's reinforced, the more reward hacking you get.