Thanks a lot, guys, seriously. Now I have to go find a script to convert a few hours email from mbox to maildir without stomping all over what’s already in the maildir folders.
You may wish to lodge a Bug Report via http://www.virtualmin.com/bug-tracker/ instead of posting in the forum to get faster and more direct response. And to have Jamie fix the issue permanently.
Apparently it is an issue regarding my postfix config. Because I didn’t have a home_mailbox = Maildir/ line in my main.cf, it decided that what I really meant was mbox style delivery, and built the procmail script in that fashion. Of course, there was nothing wrong with the old procmail config that needed rebuilding, and you don’t NEED a home_mailbox line when you’re using mailbox_command to farm off processing of mail, but apparently all this is my fault and I ought to be ashamed.