If you view a custom tag the result will be: name tag unknown. I know that putting the command $IsNull this error is not displayed but a space is inserted instead.
I created this code that allows me to enter both the disc number, if there are more than one in the album, and also the title of the grouped sub-tracks at the same time.
{color: 255,155,0} {font: Segoe UI Semibold;Bold, Italic;10}$If(<N° dischi>>1,$If($Contains(<Sottotitolo tracce>,*)="T",$IsNull(<Sottotitolo tracce>,,$CutLeft(<Sottotitolo tracce>,1)),"Disco n: "<N° disco>$IsNull(<Sottotitolo tracce>,," - "<Sottotitolo tracce>)),<Sottotitolo tracce>)
Everything works fine except that if the disk is single and has no subtracks the first row is empty causing the other tracks to be scaled down.

without $IsNull

with $IsNull
Note that there is a gap between the dividing line and the first track that normally shouldn't be there.