@Jim
I have media center set to store lyrics in the tags and the database.
The only other music software I use regularly is MusicMagic Mixer and SlimServer. MusicMagic only writes its analysis to the tags. SlimServer only reads tags. The MC plugins I use are aTagger, Chart Finder, and Lyrics Finder. However, most (if not all) of the affected songs had not been touched in a while — not by MC, plugins, or other software -- not even to play them.
It's just that out of the blue MC shows the lyrics fields as blank when the lyrics were there before. I'm worried that if I edit some other tag before noticing missing lyrics that MC will overwrite the existing lyrics in the tags with an empty lyrics tag. Is this a possibility?
@Alex
Like I said to Jim, the majority of these files had not been touched, so there would be no reason for MC to be updating the database for them. That is what's most puzzling.
I do make regular Library backups, but since I had recently added a bunch of new music to my library and I wasn't sure if the lyrics went missing before or after, I chose to do "update Library from tags" this time. I've been doing incremental backups at every folder change but maybe I need to spread them out a little more.