Why is iRedMail blocking this Mattermost site?

Question

iRedMail's login takes over other subdomain's webpage:

iRedMail 1.6.8 installed on fresh Debian 11 install with MariaDB 15.1 and nginx 1.18.0, then downloaded, installed and set up Mattermost-9.5.2-linux-amd64 using Postgresql 13.14, all on Hostinger KVM1 VPS.

I web-surfed to 'https://mm.example.com'

I expected a Mattermost page.

'https://mm.example.com' comes back as 'https://mm.example.com/mail' and looks like 'https://mail.example.com'

In other words, mm.example.com and mail.example.com look alike.