Yes. The Administrator account is a special account. It does not use Active Directory information. If Active Directory authentication is enabled, all other users must have the same name within Symantec DLP as in Active Directory.
For AD Login step by step configuration follow the link provided below:
How to Configure AD User login Authentication in Enforce for Data Loss Prevention 15.x and above (broadcom.com)
AD usernames are not case-sensitive, but Symantec DLP user names are. For example, you have an active directory user "John" that is then added to Symantec DLP. He will be able to log into his desktop as "john" or "JOHN". However, when logging into Enforce, he will always have to use the username "John". All other variations will result in authentication failure.