Yes, it's quite interesting behaviour!
Do you have FORM_INI_FILE set in FormMail's configuration?
If so, perhaps you have a specification in the INI file that's triggering this.
Otherwise, I think I will need to investigate exactly what's going on. In this case, please send me access details to your server using this secure form:
or just email me your copy of FormMail to start with (scripts-new/formmail_ho.php).