Anything in the Maildir/new subdirectory that resembles the just sent email?
I see a difference with my logs in respect to the procmail part, but I'm not sure it that's the cause.
Jan 19 06:00:08 tmpoeu01 postfix/local: 82C331AC061: to=<firstname.lastname@example.org>, orig_to=<root>, relay=local, delay=7.2, delays=1.7
/0.01/0/5.4, dsn=2.0.0, status=sent (delivered to command: /usr/bin/procmail -f-)
domain.tld is obfuscated in this example. I see in the Maildir/new (as long as I have not opened it, then it will move to the 'cur' subdir) the message sent here.
hostname -f gives on my system tmpoeu01.domain.tld, but I have specifically set it to this with the hostname command and I changed /etc/hostname. But you had this working, so I shouldn't start to look there (unless you did some DNS or hostname changes.
edit: negation typo error