Is the library containing this set configured to prefer local metadata? I’ve found that Plex is pretty accurate when this is enabled, if your tags are in excellent shape.
Do you have an app, such as MP3tag, that can show us more of the tags embedded in these files?
We need to see:
* Album Title
* Track Title
* Artist (artist of each individual track)
* Album Artist
* Track Number
* Disc Number
This How-To explains some of the things I’ll be looking for in your tags.
You said this is a new disk being added to a box set. In this case, it is very important that the discnumber is correct in every disc in the set, and the album artist is “Various Artists” throughout the set.
