Gulp 4 alpha watch only work once

Someone say this problem cause OS, specify Linux-like OS ubuntu etc, and add ** to path can fix it, like

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  gulp.task('watch', function () {
      gulp.watch( './**/test.js', gulp.series('es6') );
      console.log('Running watch...');
  }); 

But the solution does not work on me, do not know why, sad~