email spam quarantine and zimbra spam folder

the quarantined emails are stored on the Proxmox Mail Gateway, not delivered to the email server and therefore not in your Zimbra server.
 
If you do not want to use the spam quarantine (emails stored on the Mail Gateway), do not use it.

Instead, just mark the Emails - e.g. with SPAM: in the subject and deliver all to Zimbra server.
 
Or add special spam field header to your spam mail and use zimbra to tag it as spam.