Why Identity Endpoint uses the error code as the key in the ProblemDetails

[soags]

Identity Endpoint uses the error code as the key in the ProblemDetails.
But, Data Annotation uses property names as keys.
Why is this the case?

ProblemDetails by Identity Endpoint

{
    "type": "https://tools.ietf.org/html/rfc9110#section-15.5.1",
    "title": "One or more validation errors occurred.",
    "status": 400,
    "errors": {
        "DuplicateUserName": [
            "Username 'test@localhost' is already taken."
        ]
    }
}

ProblemDetails by Data Annotation

{
    "type": "https://tools.ietf.org/html/rfc9110#section-15.5.1",
    "title": "One or more validation errors occurred.",
    "status": 400,
    "errors": {
        "Username": [
            "The Username field is required."
        ]
    },
    "traceId": "00-2f8811d662105f63cd09f1734a6b350e-666ffec932e8b53f-00"
}