events in shared calendar not visible to shared user ... except for recurring events!

Steps to reproduce

I fully realise that this is going to be a difficult-to-reproduce!

1. calendar-owner: share a calendar with another nextcloud user
2. another-nextcloud-user: reload calendar web page and verify that calendar-owner's calendar is listed on the left and that recurring events in calendar-owner's calendar are shown
3. calendar-owner: add a new one-off event (whole day or time-to-time, it makes no differerence)
4. another-nextcloud-user: reload calendar web page and check if the one-off event is visible

Expected behavior

event is visible

Actual behaviour

event is not visible

Calendar app version

5.0.6

CalDAV-clients used

Firefox (many versions, including 132.0.2), Android apps ICSx5 from bitfire web engineering + Simple Calendar Pro from Simple Mobile Tool

Browser

Firefox

Client operating system

Debian 12/bookworm Linux

Server operating system

Debian 12/bookworm Linux

Web server

Apache

Database engine version

MariaDB

PHP engine version

PHP 8.2

Nextcloud version

30.0.2 (but this bug has been present for a long time)

Updated from an older installed version or fresh install

Updated from an older version

List of activated apps

www-data@nuvole:~/html$ php occ app:list
Enabled:
  - app_api: 4.0.0
  - apporder: 0.15.0
  - bruteforcesettings: 3.0.0
  - calendar: 5.0.6
  - carnet: 0.25.6
  - cloud_federation_api: 1.13.0
  - contactsinteraction: 1.11.0
  - cookbook: 0.11.2
  - dav: 1.31.1
  - deck: 1.14.2
  - federatedfilesharing: 1.20.0
  - files: 2.2.0
  - files_pdfviewer: 3.0.0
  - files_sharing: 1.22.0
  - files_trashbin: 1.20.1
  - files_versions: 1.23.0
  - firstrunwizard: 3.0.0
  - integration_mattermost: 2.0.0
  - logreader: 3.0.0
  - lookup_server_connector: 1.18.0
  - news: 25.0.3
  - nextcloud_announcements: 2.0.0
  - notifications: 3.0.0
  - oauth2: 1.18.1
  - password_policy: 2.0.0
  - photos: 3.0.2
  - privacy: 2.0.0
  - provisioning_api: 1.20.0
  - settings: 1.13.0
  - sharebymail: 1.20.0
  - side_menu: 4.0.1
  - spreed: 20.0.2
  - support: 2.0.0
  - text: 4.1.0
  - theming: 2.5.0
  - twofactor_backupcodes: 1.19.0
  - updatenotification: 1.20.0
  - user_migration: 7.0.1
  - user_status: 1.10.0
  - viewer: 3.0.0
  - webhook_listeners: 1.1.0-dev
  - workflowengine: 2.12.0
Disabled:
  - activity: 3.0.0 (installed 2.21.1)
  - admin_audit: 1.20.0
  - circles: 30.0.0 (installed 29.0.0-dev)
  - comments: 1.20.1 (installed 1.19.0)
  - dashboard: 7.10.0 (installed 7.9.0)
  - encryption: 2.18.0
  - federation: 1.20.0 (installed 1.19.0)
  - files_downloadlimit: 3.0.0 (installed 2.0.0)
  - files_external: 1.22.0
  - files_reminders: 1.3.0 (installed 1.2.0)
  - hidesidebars: 2.2.5 (installed 2.2.5)
  - recommendations: 3.0.0 (installed 2.1.0)
  - related_resources: 1.5.0 (installed 1.4.0)
  - serverinfo: 2.0.0 (installed 1.19.0)
  - survey_client: 2.0.0 (installed 1.17.0)
  - suspicious_login: 8.0.0
  - systemtags: 1.20.0 (installed 1.19.0)
  - twofactor_nextcloud_notification: 4.0.0
  - twofactor_totp: 12.0.0-dev
  - user_ldap: 1.21.0
  - weather_status: 1.10.0 (installed 1.9.0)
www-data@nuvole:~/html$

Nextcloud configuration

www-data@nuvole:~/html$ php occ config:list system
{
    "system": {
        "overwrite.cli.url": "https:\/\/nextcloud.pasta.freemyip.com",
        "htaccess.RewriteBase": "\/",
        "trusted_domains": [
            "nextcloud.pasta.freemyip.com"
        ],
        "overwritehost": "nextcloud.pasta.freemyip.com",
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "30.0.2.2",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "app_install_overwrite": [
            "apporder",
            "carnet",
            "news",
            "quickaccesssorting"
        ],
        "mail_smtpmode": "sendmail",
        "mail_sendmailmode": "smtp",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "default_phone_region": "49",
        "maintenance_window_start": 1,
        "memcache.local": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "filelocking.enabled": "true",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "port": 0,
            "timeout": 0
        },
        "maintenance": false,
        "theme": "",
        "loglevel": 2,
        "has_rebuilt_cache": true
    }
}
www-data@nuvole:~/html$

Web server error log

The calendar updates described in the recipe section above do not generate any error log entries.

Log file

www-data@nuvole:~/html$ find . -name nextcloud.log -ls
   278841      0 -rw-r-----   1 www-data www-data        0 Jun 27 09:41 ./data/nextcloud.log
www-data@nuvole:~/html$

Browser log

not sure how to do this, or, given that the problem shows up on a variety of clients, whether it makes sense to include it.

Additional info

This bug has been here for a long time and has survived several upgrades and even a complete new installation (with transfer of user data). If you need more information, then please let know how recipes/files/etc.

Start by reproducing the shared-calendar steps with Nextcloud Calendar 5.0.6, comparing recurring and one-off events for the shared user. Trace the Calendar/CalDAV handling of shared-event visibility; done means newly added one-off events are visible to the shared user while recurring events remain visible.