Convert Text File- Tag Not Changing Filename(s)

"Convert Text File -Tag " amended all tags, except filenames of the 20 mp3 files in a directory.

It would not change the filenames even the CSV file contained only the filename field.

Yes, this is so.
see e.g. /t/15832/1

The converter "Convert Text file - Tag" is able to import text data from a text file into one or more tag-fields. Simultaneous renaming of a file is not supported.