Nakaba said that Arthur chose not to regenerate his arm so he would never forget that the races would never get along, while that makes sense, if everything is analyzed further it doesn't really make sense. Arthur's personality change should have been gradual, at the end of the chaos arc the Arthur we see is still the same as he always was, he should have regenerated his arm when he took cath's chaos back. Even though he had started to change personality after the Cursed by the Light movie, still, it's months after the chaos arc, why didn't he just regenerate his arm during that time? This all implies that Arthur has always been unhappy with the relationship between the races, but that doesn't really seem to be the case.