CXone Admin API bulk skill proficiency update failing with 422

My current config is completely failing…

Attempting to bulk update agent skill proficiencies via the CXone Admin REST API. The payload structure matches the OpenAPI spec for /api/v2/admin/users/{userId}/skills, but I am consistently hitting a 422 Unprocessable Entity error. The response body indicates a validation failure on the proficiencyLevel field despite passing valid integer values. Here is the raw JSON body I am sending:

{
 "skills": [
 {
 "skillId": "12345",
 "proficiencyLevel": 5,
 "effectiveDate": "2023-10-27T00:00:00.000Z"
 }
 ]
}

The documentation states that proficiencyLevel is optional, but omitting it results in the same 422 error. Is there a hidden constraint on the effectiveDate format or a required header for bulk operations that is not documented? I have verified the OAuth token has the necessary admin:users:write scope.