Ah. Well, for IMAP, parsing into message/rfc822 is the mail server's responsibility, according to RFC 3501 (Gmail, AOL, FastMail, Yahoo -- all do this).
Kostya, I believe Cyrus Imapd (the IMAP server we're using) is correctly parsing it. My guess is that Aqua Mail is somehow incorrectly fetching/interpreting the mail parts as parsed by the server. We have not experienced issues with RFC822 forwards with any other clients, including Outlook, Thunderbird, various webmails including Roundcube and Horde IMP, Apple Mail, and also K9.
If you like, I could see if there is an example mail that I could redirect to you. Redirecting should allow to keep the mail structure intact.