Webpack changes charset in CSS styles

I have a CSS file, that I need to import into my application. And my problem is that Webpack replaces strings like "\f2e9" to unreadable thing.

For example. That's the part of the stylesheet I import:

.grid-cell.grid-cell-state-error:before {
  font: normal 22px/28px "corefonticon";
  content: "\f2e9";
}

.grid-cell.grid-cell-state-neutral:before {
  font: normal 22px/28px "corefonticon";
  content: "\f2e9";
}

And that is how it looks in the bundled CSS file:

.grid-cell.grid-cell-state-error:before {
    content: "";
    font: normal 22px/28px corefonticon;
}

.grid-cell.grid-cell-state-neutral:before {
    content: "";
    font: normal 22px/28px corefonticon;
}

In the original file I can modify every character of content. But in the bundled file it is a single character.

Here's my Webpack config:

{
  test: /(\.s[ac]ss)|(\.css)$/,
  use: [
    MiniCssExtractPlugin.loader,
    'css-loader',
    { loader: 'sass-loader', options: { sourceMap: !isProd } },
  ]
}

How can I prevent it?