It looks like the rewrites configuration for /robots.txt and /sitemap.xml don’t actually change the destination path, so they should be removed. Can you verify that those files/routes exist in the build output and deployed site?
Thanks for sharing those details. Are you able to share the public repo or a minimal reproducible example? Even just sharing some info about how routing is handled in your React app would give us more insight.
I also recommend checking the runtime logs for more info about the error. That can give you more clues about why your app is displaying a custom 404 page even though the destination paths exist in your deployment output.