Did you ever get this solved? I think I've figured out why Muse is doing it and I can fix the code. Unfortunately I'd have to fix it every time we export the site.
/scripts/form-<id>.php has a definition for the email with a 'from' and 'to' field. When I look through /scripts/form_process.php, line 128 in the get_email_headers function, it looks like it is using the 'to' variable as the 'From' field. If you have multiple email address defined in 'to', that would be a problem.