Why it does not console.log cf_callType?

urlSearchParams.js

    // Because URLSearchParams iteration omits cf_callType key when iterating, I am using this function:
    // Bug reproduction: https://gist.github.com/rofrol/e226e396d7dda5d0d566e0abb305ae2c#file-urlsearchparams_confused-js
    // https://stackoverflow.com/questions/8648892/how-to-convert-url-parameters-to-a-javascript-object/8649003#8649003
    class URLSearchParams {
      constructor(search) { this.searchParams = this.searchToSearchParams(search); }
      searchToSearchParams(search) { return JSON.parse('{"' + search.replace(/^\?/, '').replace(/&/g, '","').replace(/=/g, '":"') + '"}', (key, value) => key === "" ? value : decodeURIComponent(value)) }
      has(key) { return key in this.searchParams; }
      get(key) { return this.searchParams[key]; }
      set(key, value) { this.searchParams[key] = value; }
      delete(key) { delete this.searchParams[key]; }
      toString() {
        return Object.entries(this.searchParams).map(([key, value]) => {
          return encodeURIComponent(key) + '=' + encodeURIComponent(value)
        }).join('&');
      }
      *[Symbol.iterator]() {
        for (let tuple of Object.entries(this.searchParams)) {
          yield tuple;
        }
      }
    }

UrlSearchParams_confused.js

{
  const callFormPrefix = 'cf_';
  let searchParams = new URLSearchParams("MDN=12341234&cf_mdn=12341234&cf_callReason=R3&cf_callType=Escalation");
  console.log(searchParams.toString());
  for (let [key] of searchParams) {
    console.log('key', key)
    if (key.startsWith(callFormPrefix)) {
      console.log('starts2:', key)
      searchParams.delete(key);
    }
  }
  'cf_callType'.startsWith(callFormPrefix)
}