Is it possible that this problem can still be reproduced when Creating a read-only mailing list mirror?
We cannot get forwarded email through in a mailing list mirror category, while forwards land just fine in another category in the same test instance with posting via email enabled.
We are discussing this problem here and you can find several tests in this mailing list mirror category created to test this problem.
Maybe the solution is to skip any removal of content in mailing list mirror mode? Mirror is mirror, and the risk of missing content is way more expensive than the convenience of not showing signatures (especially when we are thinking that Discourse becomes the de facto archive of a mailing list).