Pokémon page (https://trakt.tv/shows/pokemon-1997) is completely messed up. I checked both TVDB and TMDB and realised that TMDB list of seasons/episodes is right according to the international order, but there is a problem with TVDB, because since season 16 they're using the japanese order (while seasons 1-15 follow international order). My suggestion is to sync trakt data according to TMDB.
This was fixed a while ago and uses TMDB as the data scraper now.
Brandon Marlow commented
Throwing my support behind this. This should be fixed ASAP because it's a major mess. You have 13 seasons using the international season structure and then season 14 onwards using the Japanese structure. ON TOP OF THAT you then have the newest season's episode listed under both season 19 and season 20. It's ridiculous.