Update on the experiment:
Ran a test on postman using a patch setting the username to null and the API behaves as expected.
The problem here lies on the C# SDK and the way it exposes the operations.
The SDK defines a class UserUpdateRequest that contains all user attributes.
The API docs states that if you need to remove some attribute from the user, the request should specify null as a value of the field.
Since the SDK only have a typed method using the USerUpdateRequest class, and do not expose one generic method for the patch method, it cannot be done using the current C# SDK version.
As a workaround , included a httpclient on my service and used it in conjuction with a specific object to issue the patch on the user passing null.
not an ideal solution given now i have to maintain two distinct points in my code that makes requests to the management API but it works.