Postman Public Documentation published on a custom domain may not work due to the following reasons:
1. Domain verification not done.
2. Certificates from LetsEncrypt not generated on your domain.
1. Domain Verification - Once a custom domain is added on your Postman account, it is important to complete the domain verification by creating the following DNS records:
TXT record - You need to configure on the root domain (eg: example.com)
CNAME record - need to be configured on the subdomain (eg: api.example.com or docs.example.com)
Please refer to the following article for more information - https://www.getpostman.com/docs/v6/postman/api_documentation/adding_and_verifying_custom_domains
2. Certificates from LetsEncrypt not generated on your domain:
Postman should have the authority to serve traffic on your domain. LetsEncrypt lets us serve traffic by issuing us a certificate on your domain.
If your domain has CAA records set, you must explicitly allow LetsEncrypt to set certificates for your domain. Without this, certificates generated by LetsEncrypt won't be respected by browsers. To tackle this, follow this guide - https://letsencrypt.org/docs/caa/ to add Letsencrypt to your CAA records.