Plus using the separator is really as Julian suggested (and you
confirmed) a deviant case since when piping single messages, the
separator really has no use or meaning. To quote Julian:
The initial From line is NOT part of the RFC standard, nor is it
part of the message. It's an obsolete Unix mbox hack that should
have been killed off decades ago - the mbox format is patently
unsuitable, leading to abominations like From-stuffing. I've had
immense trouble with very broken mail systems (procmail) that
insist on putting the From in where it has no business - it caused
standards-compliant mail filters to break, because they saw a From
instead of a proper message.
Even I (who is the one who requested the From_ header) don't have a use
case for it in the case of individual messages. I only need it when
piping multiple messages.