“Merge” should still be there, though it may have moved (don’t worry, I’m not really merging Abba and Paula Abdul):
I assume the new album is in the same folder structure as the rest, so:
You might try moving the new album out of the library’s folder structure, scanning the library so Plex removes it, then putting it back and scanning again (a simplified version of the “Plex Dance”). If push comes to shove, you could do that with the entire Sleater-Kinney folder.
If none of that works, I’d still suspect something to do with tags, but I’d need to see screenshots of the tags in MP3Tag.
