Domain example.com is resolved to another server and its actual IP address doesn't match the one specified in Plesk > Domains > example.com > Web Hosting Access.
As a result, DNS Helper feature shows a false-positive warning in Domains > example.com > DNS Settings:
Your website is offline because its domain name does not resolve to the server IP address.
To put your website online, update DNS settings on the side of your domain registrar or external DNS provider.
Your website will be up and running once DNS changes come into effect. It may take up to 24 hours.
An external feature request with ID #PPPM-12937 was created to change the warning's misleading phrasing.
As a workaround, the DNS Helper feature can be disabled as follows: