As per the NextJS documentation, the inclusion of the postcss.config.cjs file overrides the default NextJS CSS compilation behavior:

Warning: When you define a custom PostCSS configuration file, Next.js completely disables the default behavior. Be sure to manually configure all the features you need compiled, including Autoprefixer. You also need to install any plugins included in your custom configuration manually, i.e. npm install postcss-flexbugs-fixes postcss-preset-env.

The PostCSS configuration file in this template should be modified to something similar to keep the default behavior:

module.exports = {
  plugins: {
    "postcss-preset-mantine": {},
    "postcss-simple-vars": {
      variables: {
        "mantine-breakpoint-xs": "36em",
        "mantine-breakpoint-sm": "48em",
        "mantine-breakpoint-md": "62em",
        "mantine-breakpoint-lg": "75em",
        "mantine-breakpoint-xl": "88em",
      },
    },
    "postcss-flexbugs-fixes": {},
    "postcss-preset-env": {
      autoprefixer: {
        flexbox: "no-2009",
      },
      stage: 3,
      features: {
        "custom-properties": false,
      },
    },
  },
};

Moreover a note about this could also be included somewhere on the Mantine PostCSS Setup guide to at least inform users.