[Dovecot] pop3 not autocreating directory structure

We use:

mail_home = /srv/mailstore/%256LRHu/%Ld/%Ln
mail: mdbox:~/mdbox

and I just noticed one of our newly provisioned users initially failed to pop her mails. I saw several of these:

dovecot::  pop3(new.user@example.net): Error: Couldn't open INBOX: Mailbox doesn't exist: INBOX
dovecot::  pop3(new.user@example.net): Couldn't open INBOX top=0/0, retr=0/0, del=0/0, size=0

before she switched to imap and then everything looked fine:

dovecot::  imap(new.user@example.net): Disconnected: Logged out bytes=11/338

Looking at the timestamps in the filesystem I see that the users home directory wasn't created before switcing to imap.

Is this a know problem?

-jf