The file has "LYRICS" and "UNSYNCHEDLYRICS" as tags. I realise there is no official standard, but somewhere there was an attempt at a standard either on hyrdogenaudio or xiph.org which just had LYRICS as the tag, so thats the "official" tag MB uses. However MB does also read "UNSYNCHEDLYRICS" for wider compatibility. So for the tagged value, because you are using both then they get concatenated.
However the logic in MB gives preference to synched lyrics and for flac files, the only "offical" way is either a .lrc file or having the "LYRICS" tag set to the synched lyrics.
So if you had a .lrc file it should have worked. It did in my testing