Unable to use a custom attribute as uid

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Steps to reproduce

1. Configure an OIDC provider to expose a members unique ID as "memberid"
2. In "Attribute Mapping / User ID mapping" set "memberid"
3. Try to log in with the user

Expected behaviour

I want to use the memberid as "uid" in nextcloud.
The memberid is a 9 digit number which is exposed in the standard claims delivered by the OIDC IDP.

Actual behaviour

When selecting "login with idp" I get redirected for log in and end up with "Failed to provision the user".
I can see the attribute being part of "Parsed the JWT payload:" in the debug log.
It works when I change "memberid" to "preferred_username".

Server configuration

Web server: Apache

Database: Maria

PHP version: 8.1/8.2/8.3

Nextcloud version: 30.0.4 (also tested 30.0.12 on Hetzner Storage Box, same behaviour but no access to logs)

{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "owncloud.example.com,
            "nextcloud.example.com"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "overwrite.cli.url": "https:\/\/owncloud.example.com",
        "dbtype": "mysql",
        "version": "30.0.4.1",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "theme": "",
        "maintenance": false,
        "forcessl": true,
        "loglevel": 0,
        "trashbin_retention_obligation": "14,60",
        "mysql.utf8mb4": true,
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpmode": "smtp",
        "mail_sendmailmode": "smtp",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "587"
    }
}

Browser

Problem is not browser related

Start in the user_oidc app's attribute-mapping and user-provisioning flow, using the debug log entry “Parsed the JWT payload” and the “Failed to provision the user” path. Reproduce with a numeric memberid mapped to uid, compare it with preferred_username, and trace where the custom claim is rejected. Done means a mapped nine-digit memberid provisions the Nextcloud user successfully.