Discourse uses the reply_key to associate bounces with sent emails. When Discourse sends mail, it uses the reply_by_email_address value as the SMTP envelope sender.
You need to ensure your reply_by_email_address includes a {reply_key} so your instance can associate the bounces with the correct sent mail.
For instance, here on meta it’s set to incoming+%{reply_key}@meta.discoursemail.com and anything set to that address gets delivered to this instance.