How to enable and force Autoprefixer with Next.js?

I'm trying to enable vendor prefixes with Next.js, but they're not working. I'm using SCSS modules, and tried also with normal CSS but this is not working.

EDIT: I'm using a custom PostCSS file:

module.exports = {
    plugins: [
        "postcss-flexbugs-fixes",
        [
            "postcss-preset-env",
            {
                autoprefixer: {
                    flexbox: true,
                    grid: "autoplace",
                },
                stage: 3,
                features: {
                    "custom-properties": true,
                },
            },
        ],
    ],
};