If you add a folder of media right during the creation of a library, Plex is using the file date as ‘Date added at’. Which means that these items now have a date in the future in the plex database.
You could perform the Plex Dance with the few affected items.
To prevent this from happening in the future, configure your NAS/file server to fetch correct clock time automatically, using the ntp service.
There should also be software available for every OS, which allows you to change the file dates back to sane values.
