@ Carmilien
You're using the virtual tag:
$If(<Disc Count>>1,$IsNull(<Set Subtitle>,"• Disc "<Disc#>" •","• Disc "<Disc#>" • "<Set Subtitle>" •"),$IsNull(<Set Subtitle>,,"• "<Set Subtitle>" •"))
This will always display • Disc # • for any multi-disc album with or without a <Set Subtitle> tag.
If you want to display just the <Set Subtitle> and not the Disc#, change the vitual tag to:
$If(<Disc Count>>1,$IsNull(<Set Subtitle>,"• Disc "<Disc#>" •","• "<Set Subtitle>" •"),$IsNull(<Set Subtitle>,,"• "<Set Subtitle>" •"))