Since FLAC metadata don’t have a general standard for disk number, currently PLEX is not reading it, but lots of software, for example widely used mp3tag is using DISCNUMBER field for tagging the disk number in FLAC.
Plex does support discnumber. In fact, for multi-disc releases, Plex can’t do without it. It’s possible that having the ‘/2’ in that field is causing problems for Plex. I’ve always been told not to do that.