I had a situation recently where a client had a new Windows Server 2016 Domain and during setup, we used their main external FQDN for Forest Domain. (externaldomain.com)
On competition of the project, all was fine except they were unable to access their external website. Everything else was fine.
Normally we would setup DNS with an A record for the www entry, however the website was built with SEO in mind, and redirected www to https://externaldomain.com and some money already invested. This would not resolve internally.
We decided to install Squid-Cache