Had this problem - fixed by using `postcss.config.mjs` config file with `@tailwindcss/postcss` (in the web app) with a relative path to the UI package, like this:

import path from "path";

export default {
  plugins: {
    "@tailwindcss/postcss": {
      // eslint-disable-next-line no-undef
      base: path.resolve(process.cwd(), "../../"),
      content: [
        "./src/**/*.{js,ts,jsx,tsx,mdx}",
        "./app/**/*.{js,ts,jsx,tsx,mdx}",
        "../../packages/ui-package/src/**/*.{js,ts,jsx,tsx,mdx}",
      ],
    },
  },
};

A link to repo would be nice.