You probably used an unlucky field “separator” character. My favorite is a TAB, because it is quite unlikely to appear in actual metadata from Plex.
When opening the CSV file in your spreadsheet app, you should see some options on how to import it and which character to interpret as field separator. MS Excel only shows this kind of dialog if you “import” the file, not when you perform a regular “open file” command.
Try Libre Office instead.
