Documents marked as "Expired" not archived

I have a number of mailboxes (including my own) from which the server archiving (load compact mail/username.nsf -a) seems unable to archive documents marked as expired.

I have recently upgraded our servers to 6.5.5 FP2 but have also noticed the problem on another domain I manage that is at 7.0.2.

Has anyone else seen this problem? I have tried changing the mailfile template several times, even reverting to the R5 template and back to R6. I have checked and re-checked the archive settings too.