gulp.dest failed overriding existent files

Question

I have this simple gulp task to process .scss files:

var gulp = require('gulp'),
    compass = require('gulp-compass'),
    autoprefixer = require('gulp-autoprefixer'),
    plumber = require('gulp-plumber'),
    browserSync = require('browser-sync'),
    reload = browserSync.reload;

gulp.task('css', function() {
    gulp.src('app/scss/style.scss')
        .pipe(plumber())
        .pipe(compass({
            config_file: './config.rb',
            css: 'app/css',
            sass: 'app/scss',
            require: ['susy', 'breakpoint']
        }))
        .pipe(autoprefixer('last 2 versions'))
        .pipe(gulp.dest('app/css'))
        .pipe(reload({
            stream: true
        }));
});

My problem is that when there is already an app/css/style.css, it won't be overridden by running task gulp css.

As per https://github.com/wearefractal/vinyl-fs#destfolder-opt, gulp.dest should by default override the files in the destination folder. And I have another gulp task for uglify js, the overriding in which works well.

Could someone give me some hint on why this is happening?