PostCSS Autoprefixer not working

In this link, Laravel documentation says :

By default, Mix leverages the popular Autoprefixer plug-in to automatically apply all necessary CSS3 vendor prefixes.

But when I write some css like below and compile it using sass, the output doesn't have any prefixed css!

display: grid;
box-sizing: border-box;
transform: translate(-50%, -50%);

Here is my webpack.mix.js file configuration :

mix.sass('resources/sass/app.scss', 'public/css')
   .options({ processCssUrls: false });

I even used this configuration but again no prefixes!