ASAuthorizationProviderExtensionAuthorizationRequest.complete(httpAuthorizationHeaders:) custom header not reaching endpoint

I’m implementing a macOS Platform SSO extension using ASAuthorizationProviderExtensionAuthorizationRequest.

In beginAuthorization, I intercept an OAuth authorize request and call:

request.complete(httpAuthorizationHeaders: [
    "x-psso-attestation": signedJWT
])

I also tested:

request.complete(httpAuthorizationHeaders: [
    "Authorization": "Bearer test-value"
])

From extension logs, I can confirm the request is intercepted correctly and the header dictionary passed into complete(httpAuthorizationHeaders:) contains the expected values.

However:

the header is not visible in browser devtools the header does not appear at the server / reverse proxy

So the question is:

1. Does complete(httpAuthorizationHeaders:) support arbitrary custom headers, or only a restricted set of authorization-related headers ? Is there something that I might be missing ?

2. And if custom headers are not supported, is there any supported way for a Platform SSO extension to attach a normal HTTP header to the continued outbound request ?