My test replying directly from gmail (one post up) seems to have worked as expected. No extraneous content from the email I replied to is in my reply.
These things are notoriously hard to troubleshoot because different email clients behave differently. I’d suggest you add the -- signature in the email client you’re having trouble with, to nip it in the bud for your own purposes.
I am pretty sure this is not a bug so moving to Support.