Hello Steven,
thank you very much for your quick answer.
The problem is only met with v3.1.6585.
The tracks embedding LRC lyrics (with ar, ti, al tags) are detected as corrupted when performing a rescan, I mean :
before rescan with v3.1.6585 :

after rescan with v3.1.6585 (the tooltip on the left says that file seems to be corrupted) :

+ the lyrics could not be displayed (embedded or not, as soon as there are ar, ti, al tags at the beginning of the lyrics)
The problem disappears if I replace MusicBee v3.1.6585 with another version (and perform a rescan) or with the MusicBee31_Patched.zip version you just shared.
Hope this helps.