[Dovecot] Virtual mailboxes do not look into subfolders

Hi all,

I have virtual mailboxes set up as "Smart Folders" to show me messages
tagged with certain keywords. For example:

# ~/Maildir-virtual/fwp/dovecot-virtual File MailingLists/* KEYWORD @fwp

Messages get picked up from the File mailbox, but not for the sub-mailboxes inside the MailingLists mailbox. What am I doing wrong? Wrong separator?

And how can I debug this? Everything else is working fine.

Thanks a lot!

Paul