This only applies to Xero
Xero doesn't send the attachment with the email when the email is triggered by an external system like SortScape.
Xero are aware of the limitation and have an open feature request where you can vote on fixing this here https://xero.uservoice.com/forums/5528-accounting-api/suggestions/39240961
In the meantime, if you need the PDF attached to the invoice, please contact support and we can enable a workaround for you.