Mail read in system mailbox becomes unread in user mbox

Suppose I have received new emails in the system mailbox
(e.g. /var/spool/mail/theuser), and I read those new emails using
`mailx`. When I quit mailx using `q`, the read emails will be moved to
~/mbox. However, when I open ~/mbox using `mailx -f`, the emails that I
have just read in the system mailbox are marked as unread!

I think it is surprising that the mail I have read are marked as unread
when they are moved to my ~/mbox. I don't think the BSDs' mailx have
this behavior. Is this a bug in GNU mailutils?