Can't connect with TLS1.2/Client auth from IE/Chrome. Only works with Firefox

I have a client for which I'm developing an app. That app must connect to their server, which sits behind a NetScaler load balancer configured with TLS1.2 only and requires client certificate authentication.

When I tried to connect to that server through the NetScaler, I get the error "The underlying connection was closed: An unexpected error occurred on a send." I am doing this from a C# console app where I have set ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12. However, if I use TLS1.1 then it is able to connect.

If I try setting IE to use TLS1.2 explicitly, it also cannot connect to the site. If I set it to use TLS1.1, then the site loads fine.

Now here's the strange part: from Firefox, with TLS1.2 explicitly set, the site loads normally. I cannot for the life of me figure out why Firefox is the only way to authenticate against the NetScaler with a client certificate and TLS1.2.

BTW I tried from multiple machines in different geographic locations and had the same results each time. Here is a WireShark dump I captured of the handshake in IE and in Firefox. The only difference I found is that FF adds a field called "Certificate Verify" but not sure if that is why it works in FF but not IE.

IN FIREFOX:

IN IE:

So can anyone share a little insight as to why this might be failing? What can I do differently in my code/configuration in order to support this?