Browsersync doesn't detect file changes with Drupal Basic Theme Grunt config

Question

When I use BrowserSync directly from the command line it works fine. But when I use the Grunt file from the Drupal Basic theme, BrowserSync is not detecting changes being made to the SASS and CSS files. Whereas the SASS to CSS conversion is working fine...

EDIT: when I use 'css/base/*.css' instead of 'css/{,*/}*.css' it works. So this must be a syntax issue. Any idea for the right syntax?

/**
 * @file
 */
module.exports = function(grunt) {

  // This is where we configure each task that we'd like to run.
  grunt.initConfig({
    pkg: grunt.file.readJSON('package.json'),
    watch: {
      // This is where we set up all the tasks we'd like grunt to watch for changes.
      scripts: {
        files: ['js/source/{,*/}*.js'],
        tasks: ['uglify'],
        options: {
          spawn: false,
        },
      },
      images: {
        files: ['images/source/{,*/}*.{png,jpg,gif}'],
        tasks: ['imagemin'],
        options: {
          spawn: false,
        }
      },
      vector: {
        files: ['images/source/{,*/}*.svg'],
        tasks: ['svgmin'],
        options: {
          spawn: false,
        }
      },
      css: {
        files: ['sass/{,*/}*.{scss,sass}'],
        tasks: ['sass']
      }
    },
    uglify: {
      // This is for minifying all of our scripts.
      options: {
        sourceMap: true,
        mangle: false
      },
      my_target: {
        files: [{
          expand: true,
          cwd: 'js/source',
          src: '{,*/}*.js',
          dest: 'js/build'
        }]
      }
    },
    imagemin: {
      // This will optimize all of our images for the web.
      dynamic: {
        files: [{
          expand: true,
          cwd: 'images/source/',
          src: ['{,*/}*.{png,jpg,gif}' ],
          dest: 'images/optimized/'
        }]
      }
    },
    svgmin: {
      options: {
        plugins: [{
          removeViewBox: false
        }, {
          removeUselessStrokeAndFill: false
        }]
      },
      dist: {
        files: [{
          expand: true,
          cwd: 'images/source/',
          src: ['{,*/}*.svg' ],
          dest: 'images/optimized/'
        }]
      }
    },
    sass: {
      // This will compile all of our sass files
      // Additional configuration options can be found at https://github.com/sindresorhus/grunt-sass
      options: {
        sourceMap: true,
        // This controls the compiled css and can be changed to nested, compact or compressed.
        outputStyle: 'expanded',
        precision: 5
      },
      dist: {
        files: {
          'css/base/base.css': 'sass/base/base.sass',
          'css/components/components.css': 'sass/components/components.sass',
          'css/components/tabs.css': 'sass/components/tabs.sass',
          'css/components/messages.css': 'sass/components/messages.sass',
          'css/layout/layout.css': 'sass/layout/layout.sass',
          'css/theme/theme.css': 'sass/theme/theme.sass',
          'css/theme/print.css': 'sass/theme/print.sass'
        }
      }
    },
    browserSync: {
      dev: {
        bsFiles: {
          src : [
            'css/{,*/}*.css',
            'templates/{,*/}*.twig',
            'images/optimized/{,*/}*.{png,jpg,gif,svg}',
            'js/build/{,*/}*.js',
            '*.theme'
          ]
        },
        options: {
          watchTask: true,
          // Change this to "true" if you'd like the css to be injected rather than a browser refresh. In order for this to work with Drupal you will need to install https://drupal.org/project/link_css keep in mind though that this should not be run on a production site.
          injectChanges: true,
    proxy: "dev.localhost"
        }
      }
    },
  });
  // This is where we tell Grunt we plan to use this plug-in.
  grunt.loadNpmTasks('grunt-contrib-uglify');
  grunt.loadNpmTasks('grunt-contrib-imagemin');
  grunt.loadNpmTasks('grunt-svgmin');
  grunt.loadNpmTasks('grunt-sass');
  grunt.loadNpmTasks('grunt-contrib-watch');
  grunt.loadNpmTasks('grunt-browser-sync');
  // Now that we've loaded the package.json and the node_modules we set the base path
  // for the actual execution of the tasks
  // grunt.file.setBase('/')
  // This is where we tell Grunt what to do when we type "grunt" into the terminal.
  // Note: if you'd like to run and of the tasks individually you can do so by typing 'grunt mytaskname' alternatively
  // you can type 'grunt watch' to automatically track your files for changes.
  grunt.registerTask('default', ['browserSync','watch']);
};

Answer 1

So, I found the solution. Apparently the official Grunt pattern should be like this:

'css/**/*.css'