If I'm understanding correctly, you're seeing the HTTP Post of the SAML response in the browser SAML trace but it's not being received at your application's endpoint. The SAML library log shows the SAML authn request being sent but not the SAML response being received. Presumably this means the HTTP Post isn't being received at your assertion consumer service endpoint.
I suggest using the browser developer tools to capture the network traffic to see exactly what happens after the HTTP Post of the SAML response. The browser must end up somewhere.
If it does end up at your assertion consumer service endpoint, I suggest adding some logging to your application to see whether it's calling SAMLServiceProvider.ReceiveSSO. If it is, there should be entries in the SAML log file showing this.