Getting 200, but returned data is HTML

Hello, I have followed the instructions step by step, and when testing REST API, I am getting 200 as return code, but returned data is pure HTML (looks like a web page). Wondering if some settings are wrong, if NGNIX is interfering (although I do have some settings in .htaccess) or if the call is badly coded.
Any hint?

Javascript code:

function wordpressConnector(){

  // URL and params
  var root = 'https://mydomain.com/wp-json';
  var endpoint = '/jwt-auth/v1/token';
  var queryParams = '?username=SOMEUSER&password=APASSWORD';
   
  // parameters for url fetch
  var params = {
    'method': 'POST',
    'muteHttpExceptions': true
  };
   
  // call the API
  var response = UrlFetchApp.fetch(root + endpoint + queryParams, params);
  var data = response.getContentText(); // returns pure HTML webpage
  var code = response.getResponseCode(); // returns 200
  var headers = response.getHeaders();
}

returned headers:

{
    "x-sucuri-cache": "MISS",
    "x-proxy-cache-info": "0 NC:000000 UP:SKIP_CACHE_SET_COOKIE",
    "x-cache-enabled": "True",
    "Transfer-Encoding": "chunked",
    "Set-Cookie": "wordpress_google_apps_login=xxxxxxxxxxxxxxxxxxxxxxxx; path=/; secure; HttpOnly",
    "Content-Security-Policy": "upgrade-insecure-requests;",
    "X-XSS-Protection": "1; mode=block",
    "X-Content-Type-Options": "nosniff",
    "x-proxy-cache": "MISS",
    "x-tec-api-version": "disabled",
    "X-Frame-Options": "SAMEORIGIN",
    "Content-Encoding": "br",
    "x-httpd-modphp": "1",
    "Connection": "keep-alive",
    "Vary": "Accept-Encoding",
    "Link": "<https://mydomain.com/?p=29989>; rel=shortlink",
    "host-header": "yyyyyyyyyyyyyyyyyyyyyyyyy",
    "Content-Type": "text/html; charset=UTF-8",
    "x-sucuri-id": "14012",
    "Date": "Wed, 12 Oct 2022 09:48:42 GMT",
    "Server": "Sucuri/Cloudproxy"
}