Where oh where is the subject line hid?

I don't know about PMG, but plain Postfix doesn't record subjects of messages.
To enable it in Postfix:

1) Create or edit /etc/postfix/header_checks file and insert or uncomment the following line:
/^Subject:/ INFO

2) Edit /etc/postfix/main.cf and insert or uncomment the following line:
header_checks = regexp:/etc/postfix/header_checks

3) Restart postfix:
systemctl restart postfix

If the solution works for you, please edit the subject of this post thread and from the drop-down menu select "[SOLVED]" as the prefix of the subject.
:)