Mail read in system mailbox becomes unread in user mbox

On 2021-11-27 12:35 +0000, Sam Lee wrote:
> 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!

To clarify:

* New mail ("N") in /var/spool/mail/theuser that the user reads will
  become new mail again in ~/mbox after being automatically moved to
  ~/mbox upon quitting `mailx`.

* Unread mail ("U") in /var/spool/mail/theuser that the user reads will
  become unread mail again in ~/mbox after being automatically moved to
  ~/mbox upon quitting `mailx`.

GNU mailutils version: 3.13.