Magento 2.4.8-p2 PayPal Payflow Pro sandbox returns 503 on checkout but works in Postman

I’m testing PayPal Payflow Pro integration in Magento 2.4.8-p2

• In payment.log I can see the transaction is approved
• On checkout, the frontend sends a request to:
  https://pilot-payflowlink.paypal.com/

but the response is 503 Service Unavailable.

• If I copy the same request as cURL and run it in Postman, it works fine and returns success.
  
  Here is my current PayPal Manager configuration
  .

What I tried

• Verified credentials (Partner/Vendor/User/Password) in Magento config.
• Tested same request in Postman (works normally).
• Confirmed sandbox accounts are active.

Question

Is my Payflow Pro setup wrong ?

How should these fields be configured correctly?