Unable to modify resold user account domain name to a subdomain when using root.

Symptoms

When attempting to modify a resold user account's domain name to a subdomain of the same domain, we are receiving the following error:

The domain “subdomain.domain.tld” may not be created by “root” because “domain.tld” is already owned by “cpaneluser”. at /usr/local/cpanel/Whostmgr/Accounts/Modify.pm line 972.

Description

We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-53019. Follow this article to receive an email notification when a solution is published in the product. 

Workaround

Modify the user account's domain name via the reseller account or enable the Tweak Setting "Allow WHM users to create subdomains across accounts".