When I unpack my backups and take a look at the library.db database, there seem to be a considerable number of records with nothing in the metadata.
I don’t know where to find the actual database on the server itself, so I can’t carry out a check… but the backup itself looks to contain all the necessary data in every other field (name, sort_name, duration, favorite… but about 90% of the metadata entries are like this…
{"description":null,"review":null,"explicit":null,"images":null,"genres":[],"mood":null,"style":null,"copyright":null,"lyrics":null,"label":null,"links":null,"performers":null,"preview":null,"popularity":null,"release_date":null,"languages":null,"last_refresh":null}