[Bug]: Task synchronization issues between multiple devices

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

I'm using multiple devices which sync tasks via CalDAV (one Android phone with Tasks; one MacOS and one iPadOS device with Reminders). When I create a task on one device, or mark one as completed, the changes are synchronized to the server (and visible in the web Nextcloud Tasks app). But often, the changes are not synchronized between the CalDAV clients, so the added task won't show up on the iPad or will not be marked as completed there (this doesn't seem to happen with the Tasks app – maybe an Apple-only issue).

Perhaps this is related to #36644. Does the pruning of sync tokens affect other clients, which possibly have not yet synced all changes?

As a workaround, I can remove the CalDAV account and re-add it. The tasks are fully in sync afterwards again.

Steps to reproduce

I don't fully know the circumstances, under which the issue can be reproduced.

1. Create or edit a task using a CalDAV client.
2. Check it appears in the web interface.
3. Check if it appears in the Reminders app of an iOS/macOS device – some of the time, it does not.

Expected behavior

The changed tasks sync across all devices.

Installation method

Community Docker image

Nextcloud Server version

27

Operating system

Other

PHP engine version

PHP 8.2

Web server

Caddy

Database engine version

MariaDB

Is this bug present after an update or on a fresh install?

Upgraded to a MAJOR version (ex. 22 to 23)

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "allow_local_remote_servers": true,
        "trusted_domains": [
            "***REMOVED SENSITIVE VALUE***",
            "***REMOVED SENSITIVE VALUE***"
        ],
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "overwriteprotocol": "https",
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "27.1.3.2",
        "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": [
            "keeweb",
            "occweb",
            "files_external_gdrive",
            "documentserver_community",
            "flow_notifications",
            "workflow_script",
            "files_external_dropbox",
            "maps"
        ],
        "maintenance": false,
        "theme": "",
        "loglevel": 0,
        "default_phone_region": "DE",
        "session_lifetime": 2592000,
        "session_keepalive": true,
        "overwrite.cli.url": "***REMOVED SENSITIVE VALUE***",
        "simpleSignUpLink.shown": false,
        "memcache.local": "\\OC\\Memcache\\APCu",
        "memcache.distributed": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "port": 6379
        },
        "mail_smtpmode": "smtp",
        "mail_smtpsecure": "ssl",
        "mail_sendmailmode": "smtp",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "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": "false",
        "twofactor_enforced_groups": [],
        "twofactor_enforced_excluded_groups": [],
        "preview_imaginary_url": "http:\/\/nextcloud-imaginary:9000\/",
        "enabledPreviewProviders": [
            "OC\\Preview\\PNG",
            "OC\\Preview\\JPEG",
            "OC\\Preview\\GIF",
            "OC\\Preview\\BMP",
            "OC\\Preview\\XBitmap",
            "OC\\Preview\\MP3",
            "OC\\Preview\\Movie",
            "OC\\Preview\\TXT",
            "OC\\Preview\\MarkDown",
            "OC\\Preview\\Imaginary",
            "OC\\Preview\\OpenDocument",
            "OC\\Preview\\Krita"
        ],
        "preview_concurrency_new": 5
    }
}

List of activated Apps

Enabled:
  - activity: 2.19.0
  - calendar: 4.5.3
  - circles: 27.0.1
  - cloud_federation_api: 1.10.0
  - comments: 1.17.0
  - contacts: 5.4.2
  - contactsinteraction: 1.8.0
  - cospend: 1.5.10
  - dashboard: 7.7.0
  - dav: 1.27.0
  - federatedfilesharing: 1.17.0
  - federation: 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
  - forms: 3.3.1
  - logreader: 2.12.0
  - lookup_server_connector: 1.15.0
  - mail: 3.4.4
  - nextcloud_announcements: 1.16.0
  - notes: 4.8.1
  - notifications: 2.15.0
  - notify_push: 0.6.5
  - oauth2: 1.15.1
  - onlyoffice: 8.2.4
  - password_policy: 1.17.0
  - passwords: 2023.11.30
  - photos: 2.3.0
  - privacy: 1.11.0
  - provisioning_api: 1.17.0
  - recognize: 5.0.3
  - 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
  - systemtags: 1.17.0
  - 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
  - weather_status: 1.7.0
  - workflowengine: 2.9.0
Disabled:
  - admin_audit: 1.17.0
  - bruteforcesettings: 2.7.0 (installed 2.3.0)
  - encryption: 2.15.0 (installed 2.13.0)
  - files_external: 1.19.0 (installed 1.12.0)
  - support: 1.10.0 (installed 1.3.0)
  - suspicious_login: 5.0.0
  - user_ldap: 1.17.0
  - user_status: 1.7.0 (installed 1.0.1)

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

No response

Additional info

No response

Start with the CalDAV synchronization path and the sync-token pruning behavior referenced in issue #36644; the report names no source files or tests. Reproduce with an Android Tasks client and Apple Reminders clients by creating or completing tasks, then verify that all clients receive the changes without removing and re-adding the account.