All working fine here. Are you sure the lyrics are actually embedded in your music files and not just downloaded cached versions that MB displays in the lyrics panel only?
Yes, all the files I tested showed
Lyrics Source: Embedded In Music File in the lyrics panel alt-menu, although they are all being embedded automatically as soon a Lyrics Provider finds lyrics ("auto-retrieve and save without prompt").
To confirm that the player was not showing lyrics from cache, I opened them in an external tag editor, and this is what I got (shown in the GIF below, where Song A is "Black Mirror" and Song B is "Dancing Queen"):
- Song A, converted "on-the-fly": no lyrics;
- Song A, converted via Format Converter: has lyrics (same as the original);
- Song A, original FLAC: has lyrics;
- Song B, original MP3, sync'd without conversion: has lyrics.
I forgot to mention in the other post, and maybe it is relevant, but I'm using MusicBee v3.1.6388 and the device being synchronized is a Mass Storage device (although it also seems to happen with an Android in MTP mode but I didn't test enough to confirm).