What I am about to say could very well be incorrect, as human memory is notoriously unreliable. However, if I am recalling correctly, there are a few minor differences between an untouched FF2US ROM and one that has been loaded into FF4kster. The only thing I can recall at the moment, however, is the behavior of the Cockatrice Summon. In an untouched ROM, it only petrifies. After being loaded into FF4kster, it causes 1 HP of damage in addition to petrification. It's been a long time since I've played the untouched ROM, so I could just be misremembering. My point, though, is that FF4kster may need to be ruled out as the culprit for the Critical Hit Bug.