Did you check to see how TheTVDB has them listed and try that? I always check TheTVDB when I add new shows. I’ve had the best luck with matching to the aired order, but I know sometimes with anime and other shows absolute order can work better.
You may also need to do the Plex Dance to make it happy again. This happens all the time to me when I add media to my server and a quick fix of the files followed by the dance fixes it right up.
