I have a small nitpick. It seems like only one SETSUBTITLE is picked up for the same ALBUM + DISCNUMBER.
The below album as shown in MusicBee
seems to be grouped as “Act II: Charlatans and Saints” in Symfonium. I only have tracks from Act I to Act III in Symfonium.
This is also why I use SETSUBTITLE instead of DISCSUBTITLE.
Fwiiw: I use the below virtual tag in MusicBee for sub-headers:
$If(<Disc Count>>1,$IsNull(<Set Subtitle>,"• Disc "<Disc#>" •","• Disc "<Disc#>" • "<Set Subtitle>" •"),$IsNull(<Set Subtitle>,,"• "<Set Subtitle>" •"))