As indicated in this previous content report, this was a special episode and not an episode of the 10th season. Consequently, it is listed in the specials season.

Only ITV treated it as a special episode, Amazon Prime and many TV channels treated it as episode 9!

ITV is the original broadcaster of this series. Consequently, we should follow the way it was broadcasted by ITV.

In my opinion in tmdb could be a special episode and Episode 9 treat like example imdb and so try not to cause conflicts in Plex or similar software. https://www.imdb.com/title/tt23137188/?ref_=ttep_ep_9

It should be listed here only as it was originally broadcasted by ITV. That means that it should be a special episode.

Each database has its own set of rules.

I have only suggested a possible solution in this problem