That is expected behaviour. By default, articles are ignored for sorting.
It is controlled by the hidden server preference ArticleStrings.
https://support.plex.tv/articles/201105343-advanced-hidden-server-settings/
If you have cleared the Windows registry of Plex data, it is reset to default behaviour. Which means if you start a new database (or clear everything that was in it before), newly added items will again get “sorting titles” according to the ArticleStrings preference.