The long song names are saved in the SD card itself (or in your backup file) in the special file that I mentioned above. You can not see the long file names with the PC unless you use the SD Explorer or similar software. Windows Explorer will not see the long names.

If you still have the original back-up that you did, you should do another restore. If that fails to rebuild the long song names, your back-up is probably somehow corrupted

Most of us keep duplicate SD cards somewhere, you could start with that if all else fails.