Hi Adrian Mandile,
After running a test, to have the Quote number appear on the email subject line you will need to make sure within the Custom Form Style, the Quote subject line has:
"Quote [Quote No.] from.."
I tested having the "Quote [refnumber] " in the subject line and, it does not populate the number associated within the email. Are you using a Quote created within QuickBooks or have you imported your own Custom Form Style Quote, designed outside of QuickBooks?
I would make the change from Quote [refnumber] to Quote [Quote No.] to fix this. If you require any further assistance with these steps you can always reach out to our support team by clicking here.
Thanks,
Steven