Email sent to an address on the server, then forwarded to an external address, will not send the forwarded email as a sender at the original recipient's domain. Instead, the email is forwarded as the original sender.
In cPanel version v108, it was found that SRS doesn't add the Return-Path header with the SRS address when the server sends mail through a smarthost.
We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-41926. Follow this article to receive an email notification when a solution is published in the product.
Removing the smarthost configuration will allow SRS to function as expected.