Client certificate doesn't seem to be sent

Question

I integrate with the client for which I implement 2-way ssl, that is, I attach the client certificate to outgoing HTTP requests.

My calling client's application is written in C # and hosted in IIS, while the client is not using IIS, I think they are using something called layer 7.

I am adding a client certificate to the request in code:

request.ClientCertificates.Add(clientCertificate);

Now, if I send an HTTP request to a client with a client certificate, the request is not executed, the client certificate is not added to the request.

However, if before calling the request to the client, I call the request to a dummy application that I created and hosted in IIS, which requires SSL and client certificates, and then the request is called to the client, it passes!

It seems that the request for a bogus application (IIS hosting) caused something, but I can’t understand that! Why is this happening? Is this a Microsoft bug?