Why is the gulp-sourcemaps plugin not working?

Question

Node.js

There is such a gulpfile

var gulp = require('gulp');
var less = require('gulp-less');
var concat = require('gulp-concat');
var uglify = require('gulp-uglify');
var cssmin = require('gulp-cssmin');
var autoprefixer = require('gulp-autoprefixer');
var browserSync = require('browser-sync');
var sourcemaps = require('gulp-sourcemaps');
var htmlmin = require('gulp-htmlmin');



var path = {

  build: {
    html: 'dist/',
    js: 'dist/js',
    css: 'dist/css',
    img: 'dist/img',
    fonts: 'dist/fonts'
  },

  src: {
    html: 'app/*.html',
    js: 'app/js/*.js',
    css: 'app/css/*.css',
    img: 'app/img/**/*.*',
    fonts: 'fonts/**/*.*'
  },

  watch: {
    html: 'app/*.html',
    js: 'app/js/*.js',
    less: 'app/less/**/*.less',
    img: 'app/img/**/*.*',
    fonts: 'fonts/**/*.*'
  }
}

gulp.task('browser-sync', function() {
    browserSync.init({
        server: {
            baseDir: "dist/"
        }
    });
});

gulp.task('less', function() {
  return gulp.src('app/less/styles.less')
  .pipe(less())
  .pipe(gulp.dest('app/css/'))
  .pipe(browserSync.reload({stream: true}))
})


gulp.task('css', function() {
  return gulp.src(path.src.css)
  .pipe(sourcemaps.init())
  .pipe(concat('styles.css'))
  .pipe(cssmin())
  .pipe(autoprefixer({
    browsers: ['last 4 versions'],
    cascade: false
  }))
  .pipe(sourcemaps.write())
  .pipe(gulp.dest(path.build.css))
  .pipe(browserSync.reload({stream: true}))
})

gulp.task('html', function() {
  return gulp.src(path.src.html)
  .pipe(htmlmin())
  .pipe(gulp.dest(path.build.html))
  .pipe(browserSync.reload({stream: true}))
})


gulp.task('build', ['browser-sync'], function() {
  gulp.watch(path.watch.less, ['less', 'css']);
  gulp.watch(path.watch.html, ['html'])
})

The fact is that

sourcemaps

doesn't work as it should. I can not understand what the problem is, I tried to change the sequence of pipes in the task, but this did not lead to anything. Maybe someone faced a similar problem? I would be grateful for any hints.

Here is a screenshot of the problem itself, everything goes in one line and it is impossible to debug the code after minification.