Hello there,
I do not think it to be a feature or a bug but an improper configuration.
When creating a child domain a DNS delegation is created for you automatically in the root domain. Make sure you are a member of the root domains enterprise admin group and that the root domain is contactable.
AD domain domain.com looks to the servers responsible for com and looks for a delegation to itself, if one does not exist it tries to create one and will fail.
AD Domain subdomain.domain.com looks to the servers responsible for com and looks for a delegation to itself, if one does not exist it tries to create one and will fail.
Here are some threads which discuss the same https://social.technet.microsoft.com/Forums/en-US/c8c69bd7-625c-451d-be5c-00daec5aed65/issue-with-domain-controller-promotion-dns-delegation?forum=winservergen
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