Inconsistent behavior between transformed code and native code

[littlee]

(async () => {
  let obj = {};
  setTimeout(() => {
    console.log('timeout');
    obj = {};
  }, 1000);

  obj['a'] = await new Promise((resolve) => {
    setTimeout(() => {
      console.log('resolved');
      resolve('a');
    }, 1000);
  });
  obj['b'] = 'b';

  console.log('what is obj:', JSON.stringify(obj));
})();

when transformed with babel-transform-regenerator, the final value of obj is {a:'a',b:'b'}
link : https://babeljs.io/repl#?browsers=defaults%2C%20not%20ie%2011%2C%20not%20ie_mob%2011&build=&builtIns=false&corejs=3.21&spec=false&loose=false&code_lz=BQQwzgngdgxgBMAlHAvAPjgbwFBzgGwFMAXOAewCMArVLAXwG5c4wSAVASwFtCyBXYsCSoMOPHhhkoYMkQB0-MgHNgAcmLdeA1Yibjy1WpkbM6AGjgBGAAy3d2ZpSoBtVSFUBdWiADuIDqRQhD5wAAoATmRcHKxC4YQy-ABuhMjoWMx4rMScPPyCwuli-nCS0rKECspq8YkpACY6eiW1silq7vb65la21l1wdANOrhSetKpjTMxliZWKKqo-ABYgpDEGVABcqhYAUgDKAPIAcnJgxOEcUEocAGYQwE6I9kNIDEA&debug=false&forceAllTransforms=false&modules=false&shippedProposals=false&evaluate=true&fileSize=false&timeTravel=false&sourceType=module&lineWrap=false&presets=react%2Cstage-2&prettier=false&targets=&version=7.25.6&externalPlugins=%40babel%2Fplugin-transform-regenerator%407.24.7&assumptions=%7B%7D

when babel-transform-regenerator plugin removed, the final value of obj is {b:'b'}
link: https://babeljs.io/repl#?browsers=defaults%2C%20not%20ie%2011%2C%20not%20ie_mob%2011&build=&builtIns=false&corejs=3.21&spec=false&loose=false&code_lz=BQQwzgngdgxgBMAlHAvAPjgbwFBzgGwFMAXOAewCMArVLAXwG5c4wSAVASwFtCyBXYsCSoMOPHhhkoYMkQB0-MgHNgAcmLdeA1Yibjy1WpkbM6AGjgBGAAy3d2ZpSoBtVSFUBdWiADuIDqRQhD5wAAoATmRcHKxC4YQy-ABuhMjoWMx4rMScPPyCwuli-nCS0rKECspq8YkpACY6eiW1silq7vb65la21l1wdANOrhSetKpjTMxliZWKKqo-ABYgpDEGVABcqhYAUgDKAPIAcnJgxOEcUEocAGYQwE6I9kNIDEA&debug=false&forceAllTransforms=false&modules=false&shippedProposals=false&evaluate=true&fileSize=false&timeTravel=false&sourceType=module&lineWrap=false&presets=react%2Cstage-2&prettier=false&targets=&version=7.25.6&externalPlugins=&assumptions=%7B%7D