What could be the problem Gulp assembly stopped updating css files?

Question

css

gulpfile.js
Everything worked fine, but gulp stopped updating css files when I build the project. That is, when building, it must update the code to the latest version. But he stopped doing it with css files.
Tell me what could be the problem

let gulp = require('gulp'),
    sass = require('gulp-sass'),
    browserSync = require('browser-sync'),
    uglify = require('gulp-uglify'),
    concat = require('gulp-concat'),
    rename = require('gulp-rename'),
    del = require('del'),
    autoprefixer = require('gulp-autoprefixer');


gulp.task('clean', async function(){
  del.sync('dist')
})

gulp.task('scss', function(){
  return gulp.src('app/scss/**/*.scss')
    .pipe(sass({outputStyle: 'compressed'}))
    .pipe(autoprefixer({
      overrideBrowserslist: ['last 8 versions']
    }))
    .pipe(rename({suffix: '.min'}))
    .pipe(gulp.dest('app/css'))
    .pipe(browserSync.reload({stream: true}))
});

gulp.task('css', function(){
  return gulp.src([
    'node_modules/normalize.css/normalize.css',
    'node_modules/slick-carousel/slick/slick.css',
    'node_modules/animate.css/animate.css',
    'node_modules/hamburgers/dist/hamburgers.css'
  ])
    .pipe(concat('_libs.scss'))
    .pipe(gulp.dest('app/scss'))
    .pipe(browserSync.reload({stream: true}))
});

gulp.task('html', function(){
  return gulp.src('app/*.html')
  .pipe(browserSync.reload({stream: true}))
});

gulp.task('script', function(){
  return gulp.src('app/js/*.js')
  .pipe(browserSync.reload({stream: true}))
});

gulp.task('js', function(){
  return gulp.src([
    'node_modules/jquery/dist/jquery.js',
    'node_modules/slick-carousel/slick/slick.js'
  ])
    .pipe(concat('libs.min.js'))
    .pipe(uglify())
    .pipe(gulp.dest('app/js'))
    .pipe(browserSync.reload({stream: true}))
});

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

gulp.task('export', function(){
  let buildHtml = gulp.src('app/**/*.html')
    .pipe(gulp.dest('dist'));

  let BuildCss = gulp.src('app/css/**/*.css')
    .pipe(gulp.dest('dist/css'));

  let BuildJs = gulp.src('app/js/**/*.js')
    .pipe(gulp.dest('dist/js'));
    
  let BuildFonts = gulp.src('app/fonts/**/*.*')
    .pipe(gulp.dest('dist/fonts'));

  let BuildImg = gulp.src('app/img/**/*.*')
    .pipe(gulp.dest('dist/img'));   
});

gulp.task('watch', function(){
  gulp.watch('app/scss/**/*.scss', gulp.parallel('scss'));
  gulp.watch('app/*.html', gulp.parallel('html'))
  gulp.watch('app/js/*.js', gulp.parallel('script'))
});

gulp.task('build', gulp.series('clean', 'export'));

gulp.task('default', gulp.parallel('css' ,'scss', 'js', 'browser-sync', 'watch'));