ID3v2 has two dedicated frames for lyrics.
- USLT for unsynchronized lyrics.
- SYLT for synchronized lyrics.
Mp3tag only supports USLT under the name "UNSYNCEDLYRICS". This sounds a bit weird and inconsistent to me, as a lot of popular platforms like Minilyrics, MusicBee, JetAudio supports both the USLT and SYLT. Even though you can make it up by adding a user defined field into TXXX frame, it sometimes won't work properly as it's not the standard way according to ID3v2 standard.
May be consider adding support for SYLT for Mp3tag? (I know there are other software that supports this, but none was as good as Mp3tag). The support for SYLT does not necessarily need to have formatting/scrolling capability, simply being able to import data into SYLT frame is already a big step up.