Use a DNS propagation checker to make sure the DNS entry is configured properly: https://www.whatsmydns.net/
You can enter both the custom domain and the target domain in that tool to see if the results come up the same.
Note that 301 redirects often get cached locally in a web browser, so check
- If the unexpected response is as a result of a 301, and
- If the 301 is cached in a local web browser. Try an incognito tab or follow your browser instructions for removing cached 301 redirects for your domain.