whmcs payment confirmation email to the client not triggered upon invoice settlement

When an invoice on btcpayserver is paid, WHMCS does not trigger "Invoice Payment Confirmation" email to the client for some reason. I checked and I was manually able to trigger this email. Also all other emails work on WHMCS so no issue on WHMCS side.

Automatic invoice payment confirmation email from payment gateway modules other than btcpayserver work.

i checked whmcs activity log and found that invoices are being marked paid correctly by btcpayserver - the only issue is the notification not being triggered.

this module is too old, I am using latest WHMCS 8.7

No file or test is named. Start by reproducing invoice settlement through the BTCPay payment gateway and trace the flow that marks the WHMCS invoice paid, then compare it with other gateway modules that trigger confirmation emails. Done means a BTCPay-settled invoice sends the client's “Invoice Payment Confirmation” email on WHMCS 8.7.

Written by the indexing model from the issue text.