The behaviour you are describing is indeed not normal. The --prod options enables production mode which should take environment.prod.ts into account. Just in case I tested it again with a simple application. The values from the production environment were used in the final bundle in the dist directory.
What version of the ng tool are you using? You can check this by running ./node_modules/.bin/ng --version. If the problem persists you can send us your application at info@radzen.com.