I heard the same thing about Taiwanese guards. The source was an American married to an ethnic-mainland Taiwanese woman, who'd heard it from his in-laws.
It should be noted that the mainland Chinese who came over with Chiang in 1948 regarded the Taiwanese as collaborators with Japan, i.e. traitors. This was especially true regarding the Taiwanese "elite", who had been Japanese officials and even army officers. The mainlanders took over Taiwan in 1948, and ran things to suit themselves, so relations between the two communities were very hostile for a long time, and what people would believe was affected, I suspect. Still could be true. The Japanese military was a hierarchy of brutality, and those near the bottom would pass it on to those at the bottom, i.e. PoWs.
John Derbyshire quoted a survivor of the Nanking Massacre to similar effect. (He lived in Hong Kong for years, speaks Chinese, married a Chinese woman, and has traveled there several times.) Supposedly, the Manchurian and Chinese auxiliary troops were worse than the Japanese themselves. I don't recall whether Koreans were mentioned.