The ‘x/x’ scheme has been known to cause problems. You’re better off populating the disc number with simply ‘1,’ ‘2,’ etc. There are some other tips regarding embedded tags here:
[HowTo] Configure Plex to use embedded metadata (music)
I asked about this some time back. The reason they recommend turning it off is because they anticipate that most people won’t have put in the effort to make sure their embedded tags are accurate and complete. If your tags are in very good shape, you should have good results with ‘prefer local metadata’ enabled. I’ve certainly eliminated a lot of headaches by using local metadata.
