This is done to avoid that 30% of all English movies are accumulated under the letter T (or 20% of all German movies under the letter D).
Ask yourself: if you are thinking of a movie, are you usually putting the article in front of the title within your head?
I can only speak for myself, but I often omit these articles. So to me it is beneficial, that Plex is omitting the articles when it generates the ‘Sort Title’.
It is also usual in bibliographic sorting, to move the articles behind the title, after a comma. While that is indeed ugly, it serves the same purpose. To sort the index by the first word after the article.
Edit the https://support.plex.tv/articles/201105343-advanced-hidden-server-settings/
of your server and clear out it’s list of ArticleStrings.
This will prevent the stripping of the articles from future additions to your library.
It won’t change anything for all the items which are already in it, though
