Authentication issue with Google Apps Script deployed as a web app

I am facing HTTP 401 errors while trying to call a deployed Apps Script (as a web app, accessible to "anyone") from a second GAS with UrlFetch and a bearer in authorization header. The scripts were working fine for months until around two weeks ago. Here are two small scripts to reproduce the error.

Script A - Deployed as a web app, accessible to "Anyone".

function doGet(e) {
  var params = e.parameter.params;
  console.info("Parameters : " + JSON.stringify(e.parameter));
  return ContentService.createTextOutput("Success");
}

Script B - Calling the script A via UrlFetch

function callURL() {
  var param = {
    method      : "get",
    headers     : {"Authorization": "Bearer " + ScriptApp.getOAuthToken()},
    followRedirects : true,
    muteHttpExceptions:true,
  };
  var url = "https://script.google.com/macros/s/<script_A_deployed_url>/exec?param1=test";
  var resp = UrlFetchApp.fetch(url,param);
  if(resp.getContentText() != "Success"){
    console.info(resp.getContentText());
    throw resp.getContentText();
  }
}

Tanaike pointed me in the right direction. Apparently, some internal rules recently changed in the authentication mechanism for Apps Script deployed as a web app. For B script, the default scope with UrlFetch is https://www.googleapis.com/auth/script.external_request, but it looks like we now need at least read access to A script, which means we also need Drive scopes. In order to achieve that, you can for example have this function in B script to authorize them.

function setScope() {
  DriveApp.getRootFolder();
}