Are you ripping directly into a folder which is being “watched” by Plex?
Don’t. Always rip all tracks, rename them, check metadata in a separate folder, outside of a Plex library. Only after, that move the complete album folder into a Plex library.
In order for Plex to recognise a bunch of tracks as belonging to the same album, you need to:
put all tracks into the same folder
give all tracks identical content in the ‘AlbumArtist’ and ‘AlbumTitle’ metatags. If ‘track number’ and ‘Disc number’ meta tags are present, they should be present in all files of the album.
You let WMP rip into a folder, on your hard drive.
If you have pointed Plex to that very same folder, then weird things can happen, as you have seen.
If you don’t want to separate the folders of WMP and Plex, then do at least disable the automatic library update of Plex. To prevent Plex from scanning a half-ripped album.
(Settings - Server - Library)