I imported a playlist, selected all the 1302 .WMA files, and did Convert | Textfile - Tag with a 1302-line file, but got the completion message:
Tag extracted from text file and saved in 1281 of 1302 files.
and sure enough Explorer 'Date Modified' shows only 1281 files have been touched.
This is a bit worrying. Can anyone explain why 21 files are omitted? It is not because their tags are unchanged by the operation - Mp3tag writes the file regardless. This is 100% repeatable and I can see nothing special about the affected files.