https://en.m.wikipedia.org/wiki/ID3

Lyrics3v1[2] and Lyrics3v2[3] were tag standards implemented before ID3v2, for adding lyrics to mp3 files. The difference with ID3v2 is that Lyrics3 is always at the end of an MP3 file, before the ID3v1 tag.
https://id3.org/Lyrics3

https://id3.org/Lyrics3v2

It seems to me this is something pretty kludgey and could cause odd problems. It def is something outside of the id3 standard even if dependent in some way on it.

It would be better if apps supported a TXXX de facto standard.