TID 7001756 FLATFWD causes the contents of the mail to be deleted
The combination of Flat Forward , Global Signature , HTML only Message , Original Message "Forwarded as Attachment" without typing anything in the body causes the mail contents to be deleted. If any of the above condition is not true, then the message contents are not deleted.

It seems that this behavior comes back in GW 2012 SP2, with a modification: Forwarded as attachment works, simple forward does not works.