As I understand the difference of tag-types between FLAC and MP3 files, ...
FLAC uses VORBIS COMMENT tag, and MP3 uses ID3v2.x tag.
For the ID3v2.x tag the USLT data record is designed as ...
3-byte-language-token|64-byte-descriptor|text
(in Mp3tag the pipe symbol is the replacement character for the technical binary null value).
For the FLAC/VORBIS tag I would expect not such data structure, but only the text stream, without any language token and descriptor comment.
Therefore a FLAC's UNSYNCEDLYRICS tag-field should not carry the value coded for an ID3v2.x tag.
"eng||here_goes_lyrics" is not the right format for FLAC, ...
"here_goes_lyrics" should be correct, ...
... I think so.
See also ...
/t/11617/1
Sprach-Vorzeichen vor Lyrics in UNSYNCEDLYRICS id3tag?
Basic .txt export
How to import a file lyrics.txt
/t/6694/1
/t/13687/1
DD.20150627.1152.CEST