Unsubscribe Reason Not Working in Mailchimp when using MailChimp.Net Library

Question

I am using mailchimp.net library (https://github.com/brandonseydel/MailChimp.Net) to use mailchimp api. And everything is working fine except that the unsubscribe reason does not work when i update the member.

for example: Under certain event i am using following code to unsubscribe:

   existingMember.Status = Status.Unsubscribed;
   existingMember.UnsubscribeReason = "Contact Unsubscribed from contact page" ;
   //***Notice the Unsubscribe Reason here, i have different pages which has different unsubscribe reason***
   await _mailChimpManager.Members.AddOrUpdateAsync(listUserSubscribed, existingMember)
                            .ConfigureAwait(false);

Now, Next i created webhook in mailchimp as explained here (https://mailchimp.com/developer/marketing/guides/sync-audience-data-webhooks/#create-a-new-webhook). However the unsubscribe reason is always "manual" and it doesnt shows the note that i entered earlier (i.e. Contact Unsubscribed from contact page) .

Is there anything i am missing? or is there any other way to achieve the same?

Thanks in advance !!!