User has applied the project rule Automatically link expense entry attachments to invoices to a project, but on sending out the invoice, the attachment is not added automatically to the email.
This can happen if you added the project rule after you had created the invoice. The rule only applies to new invoices you create and not the ones that existed before you added the rule. So you should delete the invoice and recreate it. Core links the attachments to the invoice email after that.