I am sorry about the delayed response to your inquiry!
Thanks for providing further information on the matter.
Just to clarify, you have initiated a Management Client in your .NET application and when you have tried using the UserUpdateRequest class mentioned in this documentation?
It appears that the behaviour you have described so far is to be expected. In this situation, as you have mentioned in your second reply, the alternative would be to use an HttpClient as such:
var client = new HttpClient();
var request = new HttpRequestMessage(HttpMethod.Patch, "https://{tenantDomain}/api/v2/users/:id");
request.Headers.Add("Accept", "application/json");
var content = new StringContent("{\"username\":null}", null, "application/json");
request.Content = content;
var response = await client.SendAsync(request);
response.EnsureSuccessStatusCode();
Console.WriteLine(await response.Content.ReadAsStringAsync());
If you have any other questions on the matter, let me know!
Kind Regards,
Nik