Virtual folder with 'younger' not updating

I have a virtual folder set up with the following in dovecot-virtual:

INBOX INBOX.temp ALL YOUNGER 86400

The problem is that messages older than 86,400 seconds (24 hours) never disappear from the folder. Is there any fix for this apart from adding INDEX=MEMORY to the mailbox location in the config file?

I'm running Dovecot 2.2.36 on CentOS.

Regards,

-- Jonathan