A lot of detail in this thread, but the short version is that after Russia's invasion of Ukraine this route is operationally very difficult and resource intensive, so given the Joint Business arrangement it suits BA to cancel some services when aircraft or flight crew are in short supply, and put people on JAL or Cathay. Due to crew already in Japan some services will have to operate to bring them home.
https://www.flyertalk.com/forum/brit...a-service.html