From a very old and long post for plex database manipulation (pre windows era for plex) but works great every time for me. Restart PMS after running the following 3 commands in terminal assuming that you are running PMS on a Mac. You can copy-paste each command for here to terminal. This will sort them by added date (not modified). Let me know if it works for you.
cd ~/Library/Application\ Support/Plex\ Media\ Server/Plug-in\ Support/Databases
sqlite3 com.plexapp.plugins.library.db
update metadata_items set added_at=(select media_items.updated_at from media_items where media_items.metadata_item_id=metadata_items.id);
