gulp sass sourcemap pointing to wrong file

Question

I try to use sourcemaps for my scss files. But when I inspect an element in the browser, the sourcemap points to a wrong file. I can't find what is causing this issue.

gulp.task('sass', function(){
  return gulp.src('./themes/custom/' + project + '/scss/style.scss')
    .pipe(sourcemaps.init())
    .pipe(sass({
        includePaths: [
                  bourbon,
                  config.bowerDir + '/bootstrap-sass/assets/stylesheets',
                  config.bowerDir + '/font-awesome/scss'
          ],
        style: 'compressed',
        quiet: true,
      }).on('error', function (err) {
        sass.logError(err);
        this.emit('end');
      }))
    .pipe(concat('style.css'))
    .pipe(cleanCSS())
    .pipe(sourcemaps.write('.'))
    .pipe(gulp.dest('./themes/custom/' + project + '/css'))
});