This is a very interesting question. The Delphix Engine uses the NAME_REGEX standard, which is a POSIX and IEEE standard for portability of usernames. Specifically, usernames must match the regular expression NAME_REGEX="^[a-z][-a-z0-9]*\$" (case insensitive) and thus can not begin with a digit. You would receive an error from 'useradd' or 'adduser' as well.
In most implementations of 'adduser' the NAME_REGEX checking can be overridden with --force-badnames, but there is always a risk in doing so. Any code that assumes NAME_REGEX standards might break when encountering a username beginning with a digit. This includes the Delphix Engine code, which has not been tested with NAME_REGEX noncompliant usernames.
I can envision a workaround for this which preserves Delphix standards compliance while allowing us to interact with your AD configuration. I will raise this with our Engineering team. Unfortunately, this will not be a short-term solution for you.
For now, you could add additional users to AD for your Delphix users, with an alphabetic character prepended to the numeric username. For example, if a user has AD username '12345' you could add a secondary username 'd12345' for Delphix use.