When trying to open a domain that does not exist in Plesk like
or a server hostname, browser shows the content of website example.com
mail.example.com or hostname, the request gets redirected to an IP address that domain resolves to, thus, the content of IP’s default site is shown.
Set None in Tools & Settings > IP Addresses > 203.0.113.2 > Default site
In this case Plesk default page will be shown when trying to open a domain that does not exist in Plesk, or a server hostname (if it resolves to the server IP globally).