Just like navidrome it seems like Jellyfin reacts to failed mounts by emptying libraries.
…jellyfin.org/t-solved-network-not-mounted-before…

With version 10.11.0 they offer a built in backup system for db/metadata/subtitles though so once access is restored it’s easy to restore any metadata changes you’ve done to your library. (As long as you got a backup since before that is)
jellyfin.org/docs/general/…/backup-and-restore/#c…