SOLVED: I've now restored this field to the tags list by re-adding it to a different section. I don't think there was anything actually wrong other than the field somehow went missing and I just made an erroneous connection between the Lyrics? field I could see and the Lyrics field that had gone missing.
It appears Lyrics? may be a normal field under Advanced. Not sure what it does. I couldn't located any reference to it with search.
Recently, a "Lyrics?" field showed up in place of where the Lyrics field used to be and it only displays "●" for all files, regardless of whether the files contain lyrics. Files display lyrics when played, but the normal Lyrics field does not display in the tags list, even though confirmed exists and not set to hidden.
I checked my library fields and I have both a Lyrics field and this "Lyrics?" field. The ? one has an expression that displays the ● character if empty. I have tried hiding the "Lyrics?" field, but that does not result in the return of the normal Lyrics field to the editable tags.
Deleting the rogue field brings back the normal Lyrics tag, but I'm not super-comfortable leaving it deleted without knowing why it was there in the first place and whether that could break something. Anyone know what's going on here and whether deleting this Lyrics? field would have any negative consequences?