It’ll be a day or two before anything you change/add at TMDB / TVDB is available to you. The database people do their thing - then Plex’s Proxy has to update.
It’s not instant gratification - and I usually edit the title with the same stuff I just put on the database - or I wait (and I don’t wait).
Also:
Sometimes Plex gets that metadata (or no metadata) in it’s teeth and doesn’t want to let go (or update). That is when you:
… of course, that won’t do a thing if the info isn’t yet available… and I will fully admit that it’s a mystery you’ll not have the answer to until Plex can finally see the changes - and that ‘usually’ takes a day or two… we hope.
The moral of this story and exercise is that when you add to the online databases, you are doing it out of the love you have for the media and for others or ‘the future you’. There now… doesn’t that make you feel better?