401 error registering sandbox domain with Web Merchant Registration API

Hi, We recently applied for the Web Merchant Registration API and created our platform integrator ID. I am trying to register a domain in sandbox with the API:

POST https://apple-pay-gateway-cert.apple.com/paymentservices/registerMerchant

Content-Type: application/json
  {
      "domainNames": ["<URL>"],
      "partnerInternalMerchantIdentifier": "<merchant_id>",
      "partnerMerchantName": "<merchant_name>",
      "encryptTo": "<platform_integrator_id>"
    }

Client certificate is provided via curl --cert/--key; the TLS 1.3 handshake completes successfully — the server requests the client cert, our Certificate + CERT verify are accepted, and the connection is established. But the response is

{
    "statusMessage": "Payment Services Exception   Unauthorized", 
    "statusCode": "401"
}

How do I go about debugging this?