If you assure us that everything is the same - then why should it stop working all of a sudden?
What does MP3tag show in UNSYNCEDLYRICS?
Is the field spelled correctly and not e.g. UNSYNCED LYRICS?
Have you compared an old file that works with a new one, that doesn't?
Is the file type the same as before? Or are these e.g. wav files?
Have you checked the new files for consistency with the linked tools?
If you feel that is over the top for you could you then supply a file that you have tagged?