Synchronize all subscribed mailboxes in the frontend

Steps to reproduce

1. Use Nextcloud mail with Zimbra as IMAP server
2. Use sub-folders and some filtering rules that automatically put messages in certain folders
3. Open the Nextcloud mail client
4. Enable Subscribes and Sync in background for the folder
5. Receive a message in a sub-folder

Expected behavior

The mail client loads the new email and shows that there is an unread email.

Actual behavior

The new e-mail isn't loaded and the user is unaware of the new message in the sub-folder.

Mail app version

5.1.2

Nextcloud version

30.0.11

Mailserver or service

Zimbra

Operating system

Docker image on Kubernetes

PHP engine version

PHP 8.2

Nextcloud memory caching

config.php:  'memcache.local' => '\\OC\\Memcache\\APCu'
config.php:  'memcache.distributed' => '\\OC\\Memcache\\Redis'
config.php:  'memcache.locking' => '\\OC\\Memcache\\Redis'

Web server

Apache (supported)

Database

PostgreSQL

Additional info

From my debugging it seems that the front-end only sends a request to /apps/mail/api/mailboxes/xx/sync for the INBOX and the currently opened sub-folder. I guess that if you send this request for every sub-folder, the issue would be solved.

Start by tracing the frontend flow that sends requests to /apps/mail/api/mailboxes/xx/sync, comparing the INBOX and currently opened sub-folder behavior. Reproduce with subscribed sub-folders and background sync enabled, then verify that new messages in each subscribed folder are loaded and shown as unread.