Is it possible that your email without the form is going via an SMTP gateway? If so, all Notes-specific attributes will get removed when changed to SMTP format. Check the ROUTE- attributes in the received email for clues.
BTW, you cannot set the “From” field. Ever. It is a security device to disallow sender-spoofing. FROM is always set as the ID of the sender. Populate PRINCIPAL and then the email will be received “From: <PRINCIPAL>” and “SentBy: <FROM>”. This is the same artifact used in Notes mail when a mail-owner’s delegates send mail on their behalf.