Mail Quota doesn't update on backup deletion

Summary

When enabling backup for my mailboxes the used space get's added to the mailboxes quota usage. --> which is not configurable unlike for webfile backups :(

When I delete the backups, the quota usage is not updating, even after 24h the mail quota is still used by non existant backups.

One special case I'd like to point out: I'm using mdbox in my Dovecot configuration.

Steps to reproduce

1. enable mdbox format
2. create a mailbox
3. store some content in that mailbox
4. create a mailbox backup (i.e. daily)
5. see the quota usage increase by the backup
6. delete the backup and disable automatic backup creation
7. quota used won't be reduced even after 24h

Correct behaviour

After deleting the backups or at least after 24h I would expect the used quota to come down again.

Environment

Debian 10 ISPConfig version: 3.2.5