[Bug]: Mapping an external storage to a user's root breaks quota features

⚠️ 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 (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• Nextcloud Server is running on 64bit capable CPU, PHP and OS.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

When mounting an external storage to a user's root (/), several quota features stop working properly:

• Setting a quota effectively sets it in the database, but the API returns another user's quota (found in the browser's console via /ocs/v2.php/cloud/users/details?offset=0&limit=25&search=)
  
  
• Setting a quota sometimes doesn't show it in the user's files app (I've come to the conclusion that if the quota shows as unlimited in the admin panel, it won't show on the user side. But, if a quota is shown on the admin panel, even if it's the wrong one, it'll show the correct one on the user side.)
  
  
  
  Uploading a file sometimes refreshes and shows the limit again.
  
• Quota doesn't always update, even with quota_include_external_storage set to true and filesystem_check_changes set to 1.
• You can upload files past your quota, even when the frontend clearly states External storage "" is full, files cannot be updated or synced anymore!. You can still update existing files and upload new files.
  

Steps to reproduce

0. (Create multiple users? My test install has 3 users, 1 of them is the built-in admin.)
1. Add a quota limit to a user
2. Install the External Storage app, then map a Local storage from / to /mnt/ext/$user (this also works with a normal, global mapping)
3. Open the Users view, you'll notice that the quota has indeed vanished, but is still present in the database (and maybe in the user's Files app)
4. Set a different quota to another user
5. Reload the page, you'll notice that either a) the quota has vanished for both users or b) one of the two users will have a different quota than what is set in the DB.

Expected behavior

• Setting a quota in the Users view should persist and return correct values
• Said quota limit should show in the user's Files app and report correctly to the API
• Quota should update reliably when uploading a file
• Users shouldn't be allowed to get past their quota limit

Installation method

Official All-in-One appliance

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.0

Web server

Apache (supported)

Database engine version

PostgreSQL

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

Fresh Nextcloud Server install

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": {
        "memcache.local": "\\OC\\Memcache\\APCu",
        "apps_paths": [
            {
                "path": "\/var\/www\/html\/apps",
                "url": "\/apps",
                "writable": false
            },
            {
                "path": "\/var\/www\/html\/custom_apps",
                "url": "\/custom_apps",
                "writable": true
            }
        ],
        "memcache.distributed": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "password": "***REMOVED SENSITIVE VALUE***",
            "port": 6379
        },
        "overwritehost": "***HIDDEN***",
        "overwriteprotocol": "https",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "localhost",
            "***HIDDEN***"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "pgsql",
        "version": "25.0.3.2",
        "overwrite.cli.url": "https:\/\/***HIDDEN***\/",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "maintenance": false,
        "log_type": "file",
        "logfile": "\/var\/www\/html\/data\/nextcloud.log",
        "log_rotate_size": "10485760",
        "log.condition": {
            "apps": [
                "admin_audit"
            ]
        },
        "preview_max_x": "2048",
        "preview_max_y": "2048",
        "jpeg_quality": "60",
        "enabledPreviewProviders": {
            "1": "OC\\Preview\\Image",
            "2": "OC\\Preview\\MarkDown",
            "3": "OC\\Preview\\MP3",
            "4": "OC\\Preview\\TXT",
            "5": "OC\\Preview\\OpenDocument",
            "6": "OC\\Preview\\Movie",
            "0": "OC\\Preview\\Imaginary"
        },
        "enable_previews": true,
        "upgrade.disable-web": true,
        "mail_smtpmode": "smtp",
        "trashbin_retention_obligation": "auto, 30",
        "versions_retention_obligation": "auto, 30",
        "activity_expire_days": "30",
        "simpleSignUpLink.shown": false,
        "share_folder": "\/Shared",
        "tempdirectory": "\/mnt\/ncdata\/tmp\/",
        "one-click-instance": true,
        "one-click-instance.user-limit": 100,
        "one-click-instance.link": "https:\/\/nextcloud.com\/all-in-one\/",
        "htaccess.RewriteBase": "\/",
        "files_external_allow_create_new_local": true,
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "allow_local_remote_servers": true,
        "preview_imaginary_url": "http:\/\/nextcloud-aio-imaginary:9000",
        "quota_include_external_storage": true,
        "filesystem_check_changes": 1,
        "loglevel": "3"
    }
}

List of activated Apps

Enabled:
  - activity: 2.17.0
  - admin_audit: 1.15.0
  - calendar: 4.2.3
  - circles: 25.0.0
  - cloud_federation_api: 1.8.0
  - comments: 1.15.0
  - contacts: 5.1.0
  - contactsinteraction: 1.6.0
  - dashboard: 7.5.0
  - dav: 1.24.0
  - deck: 1.8.3
  - federatedfilesharing: 1.15.0
  - federation: 1.15.0
  - files: 1.20.1
  - files_external: 1.17.0
  - files_pdfviewer: 2.6.0
  - files_rightclick: 1.4.0
  - files_sharing: 1.17.0
  - files_trashbin: 1.15.0
  - files_versions: 1.18.0
  - firstrunwizard: 2.14.0
  - logreader: 2.10.0
  - lookup_server_connector: 1.13.0
  - nextcloud-aio: 0.3.0
  - nextcloud_announcements: 1.14.0
  - notifications: 2.13.1
  - notify_push: 0.5.2
  - oauth2: 1.13.0
  - password_policy: 1.15.0
  - photos: 2.0.1
  - privacy: 1.9.0
  - provisioning_api: 1.15.0
  - recommendations: 1.4.0
  - related_resources: 1.0.3
  - richdocuments: 7.1.1
  - serverinfo: 1.15.0
  - settings: 1.7.0
  - sharebymail: 1.15.0
  - support: 1.8.0
  - survey_client: 1.13.0
  - systemtags: 1.15.0
  - tasks: 0.14.5
  - text: 3.6.0
  - theming: 2.0.1
  - twofactor_backupcodes: 1.14.0
  - twofactor_totp: 7.0.0
  - user_status: 1.5.0
  - viewer: 1.9.0
  - weather_status: 1.5.0
  - workflowengine: 2.7.0
Disabled:
  - bruteforcesettings
  - encryption
  - onlyoffice: 7.6.8
  - suspicious_login
  - user_ldap: 1.15.0

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

(apart from mail-sending errors, no other errors are present in the data/nextcloud.log file)

Additional info

I tested this bug on two fresh installs, both from AIO, and the second one with the beta and develop docker tags for the AIO mastercontainer.

Start by reproducing the quota behavior through the Users view and /ocs/v2.php/cloud/users/details?offset=0&limit=25&search= while a local external storage is mapped to a user's root. Compare the stored quota with the API, Files app, and upload checks, including quota_include_external_storage and filesystem_check_changes; done means quotas persist, report consistently, update reliably, and cannot be exceeded.