CM SLA Rules - Credit Memo Previous Transaction Identifier - not appearing on Revenue lines

Hi

I'm using seeded SLA sources to default journal line descriptions.

For lines types of receivables I am getting all of the detail I need.

But where the line type is revenue some elements just don't display - eg the original Invoice Number.

My work around is to show the original transaction ID which I can also then show on the invoices but as end users aren't used to backend IDs this is just confusing.

Is there something obvious I'm missing or is there just a limitation where the original invoice number on Credit Memos won't show for revenue lines but will for receivables?