On domain creation under Subscriptions > domain.tld > Websites & domains > DNS settings the
*.webmail.domain.tld. appears which is not inside the DNS template.
The question is why this additional record is added when it is not in default template?
It is by design. While creating a domain with webmail enabled, Plesk will automatically add the record like
*.webmail.domain.tld. to the domain zone.
Additionally, if mailing list is enabled, then the record like
lists.domain.tld. will be added automatically as well.