Program vs. Backup Folders

Most likely, you at one time installed a small util I made called PMSLibShare

Above utillity created the share named Plex

However, time made above obsolete, since Plex Media Server now includes this.
See: QNAP FAQ

So to answer your Q, then NO !!!

They both point to the same place!

So to backup, use the PlexData one, since that’s the supported one by Plex, and the other simply something I cooked up when Plex didn’t had a siml solution

To remove my solution, do the following:

In your AppCenter, delete the PMSLibShare App
To remove the redundant share named Plex do the following:
In QNAP Web Administration, select Control Panel/Privilege/Shared folders
Find the shared folder named Plex and select it
Select Remove
DO NOT ENABLE THE CHECKMARK SAYING "ALSO DELETE THE DATA"

Then hit ok