Birthday calendar does not sync any event

Steps to reproduce

1. Have a birthday calendar
2. Sync it with a device, using e.g. Merkuro on Linux or DAVx5 on Android

Expected behavior

Entries in the birthday calendar are synced and visible on the client devices.

Actual behaviour

No client device actually shows anything in the birthday calendar, even though it is synchronised and even though the events are visible online.

Calendar app version

4.5.3

CalDAV-clients used

DAVx5, Merkuro

Browser

No response

Client operating system

KDE neon, Android 13

Server operating system

Debian

Web server

Apache

Database engine version

MySQL

PHP engine version

PHP 8.2

Nextcloud version

27.1.3

Updated from an older installed version or fresh install

Updated from an older version

List of activated apps

Enabled:
  - activity: 2.19.0
  - bruteforcesettings: 2.7.0
  - calendar: 4.5.3
  - cloud_federation_api: 1.10.0
  - comments: 1.17.0
  - contacts: 5.4.2
  - contactsinteraction: 1.8.0
  - dashboard: 7.7.0
  - dav: 1.27.0
  - federatedfilesharing: 1.17.0
  - files: 1.22.0
  - files_pdfviewer: 2.8.0
  - files_reminders: 1.0.0
  - files_rightclick: 1.6.0
  - files_sharing: 1.19.0
  - files_trashbin: 1.17.0
  - files_versions: 1.20.0
  - firstrunwizard: 2.16.0
  - groupfolders: 15.3.1
  - logreader: 2.12.0
  - lookup_server_connector: 1.15.0
  - nextcloud_announcements: 1.16.0
  - notes: 4.8.1
  - notifications: 2.15.0
  - oauth2: 1.15.1
  - password_policy: 1.17.0
  - photos: 2.3.0
  - provisioning_api: 1.17.0
  - qownnotesapi: 23.6.0
  - recommendations: 1.6.0
  - related_resources: 1.2.0
  - serverinfo: 1.17.0
  - settings: 1.9.0
  - sharebymail: 1.17.0
  - survey_client: 1.15.0
  - suspicious_login: 5.0.0
  - tasks: 0.15.0
  - text: 3.8.0
  - theming: 2.2.0
  - twofactor_backupcodes: 1.16.0
  - twofactor_nextcloud_notification: 3.8.0
  - twofactor_totp: 9.0.0
  - updatenotification: 1.17.0
  - viewer: 2.1.0
  - whiteboard: 0.0.3
  - workflowengine: 2.9.0
Disabled:
  - admin_audit: 1.17.0
  - calendar.bak: 2.3.3
  - circles: 27.0.1 (installed 23.1.1)
  - encryption: 2.15.0
  - federation: 1.17.0 (installed 1.13.0)
  - files_external: 1.19.0
  - mail: 3.4.4 (installed 3.4.4)
  - privacy: 1.11.0 (installed 1.7.0)
  - support: 1.10.0 (installed 1.6.0)
  - systemtags: 1.17.0 (installed 1.13.0)
  - user_ldap: 1.17.0
  - user_status: 1.7.0 (installed 1.3.1)
  - weather_status: 1.7.0 (installed 1.3.0)

Nextcloud configuration

{
    "system": {
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "localhost",
            "my.domain.com",
            "192.168.178.22"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "27.1.3.2",
        "overwrite.cli.url": "http:\/\/localhost",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "memcache.local": "\\OC\\Memcache\\Redis",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "port": 0,
            "timeout": 0
        },
        "filelocking.enabled": true,
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "maintenance": false,
        "theme": "",
        "loglevel": 3,
        "default_phone_region": "GB",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpmode": "smtp",
        "mail_sendmailmode": "smtp",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpsecure": "ssl",
        "mail_smtpauthtype": "LOGIN",
        "mail_smtpauth": 1,
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "465",
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "twofactor_enforced": "true",
        "twofactor_enforced_groups": [],
        "twofactor_enforced_excluded_groups": [
            "SOS"
        ],
        "app_install_overwrite": [
            "whiteboard"
        ]
    }
}

Web server error log

No response

Log file

No response

Browser log

No response

Additional info

I am not sure if this is an issue with Nextcloud's calend itself, but the fact that multiple clients fail to sync the calendar is what prompted me to open a bug report here. In case this turns out to be an issue with the clients, I have also opened a related one in KDE's bug tracker here: https://bugs.kde.org/show_bug.cgi?id=476786

No source files or tests are identified in the report. Reproduce with a birthday calendar and DAVx5 or Merkuro, then compare the calendar's online events with its CalDAV synchronization response; done means the birthday entries appear on client devices.