I believe this is the expected behaviour due to how VERP handling works in Discourse (see discourse/lib/email/sender.rb at f44ec18fd23960a22a5d5d397959430d6b84cb1c · discourse/discourse · GitHub) - we set the unique bounce address in the return path header.
I don’t have a full historical account of how this behaviour came to be, though the commented code indicates it’s to account for Rails behaviour.
Suggestion: I’m not too familiar with Netcore, would you be able to add reply.mamapedia.com as a recognised domain there?