proxy-authentication header missing with https

Question

I want to create a mitm proxy that can only be access by providing correct credentials:

(async () => {
    const mockttp = require('mockttp');

    // Create a proxy server with a self-signed HTTPS CA certificate:
    const https = await mockttp.generateCACertificate();
    const server = mockttp.getLocal({ https });

    // Inject 'Hello world' responses for all requests
    // Replace targets entirely with custom logic:
let counter = 0;
server.forAnyRequest().thenCallback((request) => {
   console.log(JSON.stringify(request));
    return {
        status: 200,
        // Return a JSON response with an incrementing counter:
        json: { counterValue: counter++ }
    };
});
    await server.start(8080);

    // Print out the server details:
    const caFingerprint = mockttp.generateSPKIFingerprint(https.cert)
    console.log(`Server running on port ${server.port}`);
    console.log(`CA cert fingerprint ${caFingerprint}`);
})(); // (Run in an async wrapper so we can use top-level await everywhere)

With http it works flawlessly, the proxy-authorization header is present:

curl -k -v --proxy "user:pass@127.0.0.1:8080" http://www.google.com

{
   "id":"8978f1a3-8a4f-4395-b0dc-0cf8929e760a",
   "matchedRuleId":"5a1bc167-7e34-4b0d-9f51-f8e49015b349",
   "protocol":"http",
   "httpVersion":"1.1",
   "method":"GET",
   "url":"http://www.google.com/",
   "path":"/",
   "remoteIpAddress":"::ffff:127.0.0.1",
   "remotePort":32932,
   "headers":{
      "host":"www.google.com",
      "proxy-authorization":"Basic dXNlcjpwYXNz",
      "user-agent":"curl/7.83.1",
      "accept":"*/*",
      "proxy-connection":"Keep-Alive"
   },
   "rawHeaders":[
      [
         "Host",
         "www.google.com"
      ],
      [
         "Proxy-Authorization",
         "Basic dXNlcjpwYXNz"
      ],
      [
         "User-Agent",
         "curl/7.83.1"
      ],
      [
         "Accept",
         "*/*"
      ],
      [
         "Proxy-Connection",
         "Keep-Alive"
      ]
   ],
   "tags":[
      
   ],
   "timingEvents":{
      "startTime":1663860475270,
      "startTimestamp":7655.8840999901295,
      "bodyReceivedTimestamp":7656.588100001216
   },
   "body":{
      "buffer":{
         "type":"Buffer",
         "data":[
            
         ]
      }
   }
}

Now the problem is that if it runs through https, the proxy-authorization disappears:

curl -k -v --proxy "user:pass@127.0.0.1:8080" https://www.google.com
{
   "id":"dd9f61c9-8ecb-4f94-87aa-095fd2f40da6",
   "matchedRuleId":"5a1bc167-7e34-4b0d-9f51-f8e49015b349",
   "protocol":"https",
   "httpVersion":"1.1",
   "method":"GET",
   "url":"https://www.google.com/",
   "path":"/",
   "remoteIpAddress":"::ffff:127.0.0.1",
   "remotePort":34557,
   "headers":{
      "host":"www.google.com",
      "user-agent":"curl/7.83.1",
      "accept":"*/*"
   },
   "rawHeaders":[
      [
         "Host",
         "www.google.com"
      ],
      [
         "User-Agent",
         "curl/7.83.1"
      ],
      [
         "Accept",
         "*/*"
      ]
   ],
   "tags":[
      
   ],
   "timingEvents":{
      "startTime":1663860737403,
      "startTimestamp":269786.7910999954,
      "bodyReceivedTimestamp":269787.29159998894
   },
   "body":{
      "buffer":{
         "type":"Buffer",
         "data":[
            
         ]
      }
   }
}

Is there anything I'm unaware of that causes this behaviour?