I also noticed (a couple of weeks ago) that I suddenly had 136 tracks with no embedded 'Album Art' AND 'Lyrics' (I tried to do a library update with no success) . I am fussy with both options and know all of my tracks have them.
I fired up my trusty standby Mp3tag and sure enough I could see the art and lyrics. To solve my problem I loaded all the suspect tracks into MP3tag and selected all of them and clicked the save button, this caused mp3tag to write to all of them.
I then re-ran the MC update and it found 136 tracks with external changes and the art & lyrics were back.
My collection (including MC database) is on a Linux server.
I'm not sure why this happened but have noticed with the new build that i get a warning about the library update is running, when i want to close MC. ( Have never seen this in a previous build) Maybe in older versions this screwed up somewhere.