While I can’t answer for which one Plex knows is correct, I can definitely say - as I got highly confised by this - that Plex has used TMDB in my library and labeled the episode “Good Bye, Sarah Jane” despite the episode on disk being labeled as “Revolution of the Daleks”.
I’m also interested to hear how Plex deals with discrepancies like this or whether Doctor Who now falls under the same category as Firefly in times of alternate episode ordering?